Twin-arginine translocation in Bacillus

ABSTRACT

Described herein are methods to enhance protein secretion in a host cell. In preferred embodiment, the host cell is a gram-positive microorganism such as a  Bacillus . In another preferred embodiment, the host cell is a gram-negative microorganism. Preferably the gram-negative microorganism is an  Escherichia coli  or a member of the genus  Pantoea . Protein secretion may be enhanced by the overexpression of protein components of the Tat pathway. Alternatively, secretion of foreign proteins can be selectively enhanced by forming a chimeric polypeptide comprising a tat signal sequence and the protein of interest. In a preferred embodiment, the tat signal sequence is selected from phoD or LipA.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of Ser. No. 09/954,737, now U.S. Pat. No.7,316,924, filed on Sep. 17, 2001, which application claims benefit ofand priority to U.S. Application No. 60/233,610, entitled “Twin-ArginineTranslocation in Bacillus”, filed Sep. 18, 2000 by Jongbloed et al.

FIELD OF THE INVENTION

The present invention generally relates to expression of proteins in ahost cell. The present invention provides expression vectors, methodsand systems for the production of proteins in a host cell.

BACKGROUND OF THE INVENTION

Eubacteria export numerous proteins across the plasma membrane intoeither the periplasmic space (Gram-negative species), or the growthmedium (Gram-positive species). The Gram-positive eubacterium Bacillussubtilis and, in particular, its close relatives Bacillusamyloliquefaciens and Bacillus licheniformis are well known for theirhigh capacity to secrete proteins (at gram per liter concentrations)into the medium. This property, which allows the efficient separation of(secreted) proteins from the bulk cytoplasmic protein complement, hasled to the commercial exploitation of the latter bacilli as important“cell factories.” Despite their high capacity to secrete proteins ofGram-positive origin, the secretion of recombinant proteins fromGram-negative eubacterial or eukaryotic origin by Bacillus species isoften inefficient. This can be due to a variety of (potential)bottlenecks in the secretion pathway, such as poor targeting to themembrane, pre-translocational folding, inefficient translocation, slowor incorrect post-translocational folding of the secretory protein, andproteolysis. Notably, many of these problems relate to the specificproperties of the general secretory (Sec) pathway that was, so far, usedin all documented attempts to apply bacilli for the secretion ofheterologous proteins of commercial or biomedical value.

General strategies for the secretion of heterologous proteins by bacilliare based on the in-frame fusion of the respective protein with anamino-terminal signal peptide that directs this protein into theSec-dependent secretion pathway. Upon translocation across the membrane,the signal peptide is removed by a signal peptidase, which is aprerequisite for the release of the translocated protein from themembrane, and its secretion into the medium. As exemplified with humaninterleukin-3, which is secreted by B. licheniformis at gram per literconcentrations, this strategy allows protein production at commerciallysignificant levels.

Two major hurdles have been identified for the secretion of heterologousproteins via the Sec-dependent route. The first one is the translocationprocess by the Sec machinery, which is composed of a proteinaceouschannel in the membrane (consisting of SecY, SecE, SecG and SecDF-YrbF)and a translocation motor (SecA). The Sec machinery is known to ‘thread’its substrates in an unfolded state through the membrane. Consequently,this machinery is inherently incapable of translocating proteins thatfold in the cytosol. A second bottleneck has been identified for otherheterologous proteins that are translocated correctly but fold slowly orincorrectly in the cell wall environment, probably because thiscompartment lacks the appropriate chaperone molecules to assist in theirfolding. Molecular chaperones of the Hsp60 and Hsp70 classes areessential for the folding of many proteins, but these are all absentfrom bacterial extracytoplasmic compartments. As the membrane-cell wallenvironment of bacilli is highly proteolytic, slowly or incorrectlyfolding translocated proteins are often degraded before being secretedinto the medium. Consequently, protein secretion via the Sec pathway isa highly efficient tool for the production of only a subset ofheterologous proteins.

Protein production and secretion from Bacillus species is a majorproduction tool with a market of over $1 billion per year. However, asnoted above, the standard export technologies, based on thewell-characterized general secretory (Sec) pathway, are frequentlyinapplicable for the production of proteins. Thus, it would bebeneficial to have an alternative mechanism for the production andsecretion proteins.

SUMMARY OF THE INVENTION

Provided herein are methods for the production of peptides in a hostcell.

In one aspect of the invention, the host cell is a gram-positivemicro-organism. The gram-positive microorganism is preferably a memberof the genus Bacillus. In a more preferred embodiment the host cell isBacillus subtilis.

In another aspect of the invention, the host cell is a gram-negativemicro-organism. The gram-negative microorganism is preferably a memberof the genus Pantoea, preferably Pantoea citrea. The gram-negativemicroorganism is preferably Escherichia coli.

The present invention also provides methods for increasing secretion ofproteins from host microorganisms. In one embodiment of the presentinvention, the protein is homologous or naturally occurring in the hostmicroorganism. In another embodiment of the present invention, theprotein is heterologous to the host microorganism. Accordingly, thepresent invention provides a method for increasing secretion of aprotein in a host cell using an expression vector comprising nucleicacid tatCd wherein said tatCd is under the control of expression signalscapable of expressing said secretion factor in a host microorganism;introducing the expression vector into a host microorganism capable ofexpressing said protein and culturing said microorganism underconditions suitable for expression of said secretion factor andsecretion of said protein.

The present invention provides expression vectors and host cellscomprising a nucleic acid encoding a TatCd and/or TatA. In oneembodiment of the present invention, the host cell is geneticallyengineered to produce a desired protein, such as an enzyme, growthfactor or hormone. In yet another embodiment of the present invention,the enzyme is selected from the group consisting of proteases,carbohydrases including amylases, cellulases, xylanases, and lipases;isomerases such as racemases, epimerases, tautomerases, or mutases;transferases, kinases and phophatases acylases, amidases, esterases,oxidases. In a further embodiment the expression of the secretion factorTatCd is coordinated with the expression of other components of thesecretion machinery. Preferably other components of the secretionmachinery, i.e., TatA and/or other secretion factors identified in thefuture are modulated in expression at an optimal ratio to TatCd. Forexample, it may be desired to overexpress multiple secretion factors inaddition TatCd for optimum enhancement of the secretion machinery.

The present invention also provides a method of identifying homologousgram positive microorganism TatCd that comprises hybridizing part or allof B. subtilis TatCd nucleic acid shown in FIG. 1 with nucleic acidderived from gram-positive microorganisms. In one embodiment, thenucleic acid is of genomic origin. In another embodiment, the nucleicacid is a cDNA. The present invention encompasses novel gram-positivemicroorganism secretion factors identified by this method.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description. It should beunderstood, however, that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the scope and spirit of the invention will becomeapparent to one skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Tat components of B. subtilis and E. coli. The amino acidsequences of Tat components of B. subtilis and E. coli as deduced fromthe SubtiList (http://bioweb.pateur.fr/Genolist/SubtiList.html) andColibri (http:/bioweb.pasteur.fr/GenolisVColibri.html) databases wereused for comparisons. Identical amino acids [*], or conservativereplacements [.] are marked. Putative transmembrane segments, indicatedin gray shading, were predicted with the TopPred2 algorithm (34, 35) (A)Comparison of TatAc (YnzA), TatAd (YczB) and TatAy (YdiI) of B. subtilis(Bsu) with TatA, TatB and TatE of E. coli (Eco). (B) Comparison of TatCd(YcbT) and TatCy (YdiJ) of B. subtilis with TatC of E. coli.

FIG. 2. The tatAC regions of B. subtilis and E. coli. (A) Chromosomalorganization of the B. subtilis tatAd-tatCd and tatAy-tatCy regions(adapted from the SubtiList database). Note that the tatAd and tatCdgenes are located downstream of the phoD gene. (B) Chromosomalorganization of the E. coli tatABCD region (adapted from the Colibridatabase).

FIG. 3. Construction of tatC mutant strains of B. subtilis. (A)Schematic presentation of the construction of B. subtilis ΔtatCd and B.subtilis ΔtatCy. The chromosomal tatCd gene was disrupted with akanamycin resistance marker (Km^(r) by homologous recombination. To thispurpose, B. subtilis 168 was transformed with plasmid pJCd2, whichcannot replicate in B. subtilis, and contains a mutant copy of the tatCdgene with an internal Bcll-AccI fragment replaced by a Km^(r) marker.The chromosomal tatCy gene was disrupted with a spectinomycin resistancemarker (Sp^(r)) by homologous recombination. To this purpose, B.subtilis 168 was transformed with plasmid pJCy2, which cannot replicatein B. subtilis, and contains a mutant copy of the tatCy gene with aSp^(r) marker in the PstI site. Only restriction sites relevant for theconstruction are shown. tatCd′, 5′ end of the tatCd gene; ‘tatCd, 3’ endof the tatCd gene; tatCy′, 5′ end of the tatCy gene; ‘tatCy, 3’ end ofthe tatCy gene. (B) Schematic presentation of the tatCd region of B.subtilis ItatCd. By a Campbell-type integration of thepMutin2-derivative pMICd1 into the B. subtilis 168 chromosome, the tatCdgene was placed under the control of the IPTG-dependent Pspac promoter,which can be repressed by the product of the lacI gene. Simultaneously,the spoVG-lacZ reporter gene of pMutin2 was placed under thetranscriptional control of the tatCd promoter region. PCR-amplifiedregions are indicated with black bars. Ori pBR322, replication functionsof pBR322; Ap^(r), ampicillin resistance marker; Em^(r), erythromycinresistance marker; tatCd′, 3′ truncated tatCd gene; T1T2,transcriptional terminators on pMutin2. (C) Schematic presentation ofthe tatCy region of B. subtilis ItatCy. By a Campbell-type integrationof the pMutin2-derivative pMICy1 into the B. subtilis 168 chromosome,the tatCy gene was placed under the control of the IPTG-dependent Pspacpromoter. Simultaneously, the spoVG-lacZ reporter gene of pMutin2 wasplaced under the transcriptional control of the tatCy promoter region.tatCy′, 3′ truncated tatCy gene.

FIG. 4. TatCd is required for secretion of PhoD. B. subtilis 168(parental strain), B. subtilis ΔtatCd, B. subtilis ΔtatCy, or B.subtilis ΔtatCd-ΔtatCy were grown under conditions of phosphatestarvation, using LPDM medium. To study the secretion of PhoD (A) orPhoB (B), B. subtilis cells were separated from the growth medium bycentrifugation. Secreted PhoD and PhoB in the growth medium werevisualized by SDS-PAGE and Western blotting, using PhoD- orPhoB-specific antibodies. (C) Cells of B. subtilis 168 and B. subtilisItatCd-ΔtatCy were grown under conditions of phosphate starvation, inLPDM medium. Next, cells and growth medium were separated bycentrifugation, and PhoD was visualised by SDS-PAGE and Westernblotting, using PhoD-specific antibodies.

FIG. 5. Two-dimensional gel electrophoretic analysis of theTatC-dependent secretion of PhoD. B. subtilis 168 or B. subtilisΔtatCd-ΔtatCy, were grown under conditions of phosphate starvation inLPDM medium. Secreted proteins were analysed by two-dimensional gelelectrophoresis as indicated in the Experimental Procedures section. Thenames of proteins identified by mass spectrometry are indicated.

FIG. 6. TatC-dependent secretion of the B. subtilis lipase LipA. B.subtilis 168 (parental strain), B. subtilis ΔtatCd, B. subtilis ΔtatCy,or B. subtilis ΔtatCd-ΔtatCy were grown in TY-medium to end-exponentialgrowth phase. To study the secretion of LipA, B. subtilis cells wereseparated from the growth medium by centrifugation. Proteins in thegrowth medium were concentrated 20-fold upon precipitation withtrichloroacetic acid, and samples for polyacrylamide gel electrophoresis(SDS-PAGE) were prepared. Secreted LipA in the growth medium wasvisualized by SDS-PAGE and Western blotting, using LipA-specificantibodies.

FIG. 7. Predicted twin-arginine (RR-)signal peptides of B. subtilis. Thelisted signal peptides contain, in addition to the twin-arginines, atleast one other residue of the consensus sequence (R-R-X-{tilde over(φ)}φ; printed in bold). The number of residues in the N- and H-domainsof each signal peptide, and the average hydrophobicity (h) of each ofthese domains, as determined by the algorithms of Kyte and Doolittle(Kyte, J., and R. F. Doolittle [1982] A simple method for displaying thehydropathic character of a protein. J. Mol. Biol. 157:105-32), areindicated. Furthermore, the RR-motifs in the N-domain, and SPase Irecognition sites in the C-domain (ie. positions −3 to −1 relative tothe predicted SPase cleavage site) are shown. Proteins lacking a(putative) SPase I cleavage site, some of which contain additionaltransmembrane domains, are indicated with ^(“TM”). One proteincontaining cell wall binding repeats is indicated with ^(“w”).

FIG. 8. Processing of prePhoD in E. coli TG1. (A) E. coli TG 1I carryingplasmid pARphoD, encoding wild type PhoD was grown in M9 minimal mediumto early logarithmic phase. 1 hour prior labelling expression of phoDwas induced with IPTG (1 mM). Cells were labelled for 1 min with[35S]-methionine, after which non-radioactive methionine was added.Samples were withdrawn at chase times 10, 20, 40 and 60 min andsubjected to immunoprecipitation with monospecific antibodies againstPhoD, followed by SDS-PAGE using a 10% polyacrylamide gel andfluorography. M, molecular weight marker; Glu, uninduced control. (B) Invivo protease mapping of PhoD in E. coli TG1 (pAR3phoD). Cells wereconverted to spheroplasts and treated with proteinase K, proteinase Kand Trition X-100 or remained untreated as indicated. Localisation ofprePhoD is indicated. Accessibility of proteinase K to the cytosol wasanalysed by monitoring SeeB in a 15% polyacrylamide gel. PhoD and SecBwere detected by monospecific antibodies.

FIG. 9. Induction and processing Of SP_(Bla)-PhoD in E. coli TG1. (A) E.coli TG1 (pMUTIN2bla-phoD) was grown in TY medium to logarithmic growthphase. Expression of bla-phoD was induced with IPTG (1 mM, lanes 2-4) orremained uninduced (lane 1). At the time of induction cultures weretreated with sodium azide (3 mM, lane 3), with nigericin (1 μM, lane 4)or remained untreated (lane 2). Samples were taken 20 min afterinduction of SP_(Bla)-PhoD, lysed and cell extracts were analysed bySDS-PAGE using 10% polyacrylamide. B and C, TG1 (pMUTIN2bla-phoD) wasgrown in M9 minimal medium to early logarithmic phase. 1 hour priorlabelling expression of phoD was induced with IPTG (1 mM). While oneculture remained untreated (B), the other was treated with sodium azide(3 mM) upon induction (C). Cells were labelled for 1 min with[35S]-methionine, after which non-radioactive methionine was added.Samples were withdrawn at times after chase as indicated in the figuresand subjected to immunoprecipitation with antibodies against PhoD,followed by SDS-PAGE using a 12.5% polyacrylamide gel and fluorography.Localisation of SP_(Bla)-PhoD and mature PhoD is indicated.[14C]-labelled molecular weight marker.

FIG. 10. Localisation of SP_(PhoD)-LacZ in E. coli TG1 in absence orpresence of B. subtilis tatAd/Cd. E. coli TG1 strains carrying eitherplasmid pAR3phoD-lacZ• (A) or plasmids pAR3phoD-lacZ, pREP4 andpQE9tatAd/Cd (B) were grown in TY medium to exponential growth andexpression of phoD-lacZ and tatAd/Cd were induced for 1 hour witharabinose (0.2%) and IPTG (1 mM), respectively. Subcellular localisationof SP_(PhoD)-LacZ was detected by in vivo protease mapping according toFIG. 8B. SP_(PhoD)-LacZ and SecB were monitored by antisera against LacZand SecB. Bands representing SP_(PhoD)-LacZ, LacZ and SecB areindicated.

FIG. 11. Processing of SP_(PhoD)-LacZ in E. coli TG1 co-expressing B.subtilis tatAd/Cd. E. coli strains TG1 (pAR3phoD-lacZ) (A) and TG1(pAR3phoD-lacZ, pREP4, pQE9tatAd/Cd) (B) were grown in M9 minimal mediumto early logarithmic phase and labelled for 1 min With [35S]-methionineand subsequently chased with non-radioactive methionine. Samples weretaken at the indicated chase times and further processed byimmunoprecipitation with antiserum against LacZ, followed by SDS-PAGEusing a 7.5% polyacrylamide gel and fluorography. Bands representingSP_(PhoD)-LacZ and LacZ are indicated. M, [14C]-labelled molecularweight marker.

FIG. 12 TatAd/Cd-mediated transport of SP_(PhoD)-LacZ in E. coli isΔpH-dependent. E. coli TG1 (pAR3phoD-lacZ, pREP4, pQE9tatAd/Cd) wasgrown in TY medium to exponential growth, nigericin (1 uM) (A) or sodiumazide (3. mM) (B) were added to the cultures prior induction of geneexpression. Localisation of LacZ was analysed by in vivo proteasemapping as described in FIG. 10. Samples were submitted to immunologicaldetection of LacZ with specific antibodies. Bands representingSP_(PhoD)-LacZ, LacZ and SecB are indicated.

FIG. 13 Localisation of SP_(PhoD)-LacZ in E. coli strain depleted fortatABCDE. E. coli strain TG1Δtat, 4BCDE (pAR3phoD-lacZ, pREP4 andpQE9tatAd/Cd) was grown in TY medium, synthesis of SP_(PhoD)-LacZ andTatAd/Cd were induced and subjected to in vivo protease mapping asdescribed in FIG. 10. LacZ and SecB were visualised by SDS-PAGE andWestern blotting,

FIG. 14 Homologs of B. clausii. B. subtilis sequences were used to BLASTsearch an in-house database of B. clausii genome.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in detail by way of reference onlyusing the following definitions and examples. All patents andpublications, including all sequences disclosed within such patents andpublications, referred to herein are expressly incorporated byreference.

Unless defined otherwise herein, all technical and scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which this invention belongs. Singleton, et al.,DICTIONARY OF MICROBIOLOGY AND MOLECULAR BIOLOGY, 2D ED., John Wiley andSons, New York (1994), and Hale & Marham, THE HARPER COLLINS DICTIONARYOF BIOLOGY, Harper Perennial, N.Y. (1991) provide one of skill with ageneral dictionary of many of the terms used in this invention. Althoughany methods and materials similar or equivalent to those describedherein can be used in the practice or testing of the present invention,the preferred methods and materials are described. Numeric ranges areinclusive of the numbers defining the range. Unless otherwise indicated,nucleic acids are written left to right in 5′ to 3′ orientation; aminoacid sequences are written left to right in amino to carboxyorientation, respectively. Practitioners are particularly directed toSambrook et al., 1989, and Ausubel F M et al., 1993, for definitions andterms of the art. It is to be understood that this invention is notlimited to the particular methodology, protocols, and reagentsdescribed, as these may vary.

The headings provided herein are not limitations of the various aspectsor embodiments of the invention which can be had by reference to thespecification as a whole. Accordingly, the terms defined immediatelybelow are more fully defined by reference to the specification as awhole.

As used herein, the genus Bacillus includes all members known to thoseof skill in the art, including but not limited to B. subtilis, B.licheniformis, B. lentus, B. brevis, B. stearothermophilus, B.alkalophilus, B. amyloliquefaciens, B. clausii, B. coagulans, B.circulans, B. lautus and B. thuringiensis.

The term “polypeptide” as used herein refers to a compound made up of asingle chain of amino acid residues linked by peptide bonds. The term“protein” as used herein may be synonymous with the term “polypeptide”or may refer, in addition, to a complex of two or more polypeptides.

The term “chimeric polypeptide” and “fusion polypeptide” are usedinterchangeably herein and refer to a signal peptide from phoD or lipAlinked to the protein of interest or heterologous protein.

A “signal peptide” as used herein refers to an amino-terminal extensionon a protein to be secreted. Nearly all secreted proteins use anamino-terminal protein extension which plays a crucial role in thetargeting to and translocation of precursor proteins across the membraneand which is proteolytically removed by a signal peptidase during orimmediately following membrane transfer.

As used herein, a “protein of interest” or “polypeptide of interest”refers to the protein to be expressed and secreted by the host cell. Theprotein of interest may be any protein which up until now has beenconsidered for expression in prokaryotes. The protein of interest may beeither homologous or heterologous to the host. In the first caseoverexpression should be read as expression above normal levels in saidhost. In the latter case basically any expression is of courseoverexpression.

The terms “isolated” or “purified” as used herein refer to a nucleicacid or amino acid that is removed from at least one component withwhich it is naturally associated.

As used herein, the term “heterologous protein” refers to a protein orpolypeptide that does not naturally occur in a host cell. Examples ofheterologous proteins include enzymes such as hydrolases includingproteases, cellulases, amylases, other carbohydrases, and lipases;isomerases such as racemases, epimerases, tautomerases, or mutases;transferases, kinases and phophatases. The heterologous gene may encodetherapeutically significant proteins or peptides, such as growthfactors, cytokines, ligands, receptors and inhibitors, as well asvaccines and antibodies. The gene may encode commercially importantindustrial proteins or peptides, such as proteases, carbohydrases suchas amylases and glucoamylases, cellulases, oxidases and lipases. Thegene of interest may be a naturally occurring gene, a mutated gene or asynthetic gene.

The term “homologous protein” refers to a protein or polypeptide nativeor naturally occurring in a host cell. The invention includes host cellsproducing the homologous protein via recombinant DNA technology. Thepresent invention encompasses a host cell having a deletion orinterruption of the nucleic acid encoding the naturally occurringhomologous protein, such as a protease, and having nucleic acid encodingthe homologous protein re-introduced in a recombinant form. In anotherembodiment, the host cell produces the homologous protein.

The term “nucleic acid molecule” includes RNA, DNA and cDNA molecules.It will be understood that, as a result of the degeneracy of the geneticcode, a multitude of nucleotide sequences encoding a given protein suchas TatC and/or TatA may be produced. The present invention contemplatesevery possible variant nucleotide sequence, encoding TatC and/or TatA,all of which are possible given the degeneracy of the genetic code.

A “heterologous” nucleic acid construct or sequence has a portion of thesequence which is not native to the cell in which it is expressed.Heterologous, with respect to a control sequence refers to a controlsequence (i.e. promoter or enhancer) that does not function in nature toregulate the same gene the expression of which it is currentlyregulating. Generally, heterologous nucleic acid sequences are notendogenous to the cell or part of the genome in which they are present,and have been added to the cell, by infection, transfection,microinjection, electroporation, or the like. A “heterologous” nucleicacid construct may contain a control sequence/DNA coding sequencecombination that is the same as, or different from a controlsequence/DNA coding sequence combination found in the native cell.

As used herein, the term “vector” refers to a nucleic acid constructdesigned for transfer between different host cells. An “expressionvector” refers to a vector that has the ability to incorporate andexpress heterologous DNA fragments in a foreign cell. Many prokaryoticand eukaryotic expression vectors are commercially available. Selectionof appropriate expression vectors is within the knowledge of thosehaving skill in the art.

Accordingly, an “expression cassette” or “expression vector” is anucleic acid construct generated recombinantly or synthetically, with aseries of specified nucleic acid elements that permit transcription of aparticular nucleic acid in a target cell. The recombinant expressioncassette can be incorporated into a plasmid, chromosome, mitochondrialDNA, plastid DNA, virus, or nucleic acid fragment. Typically, therecombinant expression cassette portion of an expression vectorincludes, among other sequences, a nucleic acid sequence to betranscribed and a promoter.

As used herein, the term “plasmid” refers to a circular double-stranded(ds) DNA construct used as a cloning vector, and which forms anextrachromosomal self-replicating genetic element in many bacteria andsome eukaryotes.

As used herein, the term “selectable marker-encoding nucleotidesequence” refers to a nucleotide sequence which is capable of expressionin mammalian cells and where expression of the selectable marker confersto cells containing the expressed gene the ability to grow in thepresence of a corresponding selective agent.

As used herein, the term “promoter” refers to a nucleic acid sequencethat functions to direct transcription of a downstream gene. Thepromoter will generally be appropriate to the host cell in which thetarget gene is being expressed. The promoter together with othertranscriptional and translational regulatory nucleic acid sequences(also termed “control sequences”) are necessary to express a given gene.In general, the transcriptional and translational regulatory sequencesinclude, but are not limited to, promoter sequences, ribosomal bindingsites, transcriptional start and stop sequences, translational start andstop sequences, and enhancer or activator sequences.

“Chimeric gene” or “heterologous nucleic acid construct”, as definedherein refers to a non-native gene (i.e., one that has been. introducedinto a host) that may be composed of parts of different genes, includingregulatory elements. A chimeric gene construct for transformation of ahost cell is typically composed of a transcriptional regulatory region(promoter) operably linked to a heterologous protein coding sequence,or, in a selectable marker chimeric gene, to a selectable marker geneencoding a protein conferring antibiotic resistance to transformedcells. A typical chimeric gene of the present invention, fortransformation into a host cell, includes a transcriptional regulatoryregion that is constitutive or inducible, a signal peptide codingsequence, a protein coding sequence, and a terminator sequence. Achimeric gene construct may also include a second DNA sequence encodinga signal peptide if secretion of the target protein is desired.

A nucleic acid is “operably linked” when it is placed into a functionalrelationship with another nucleic acid sequence. For example, DNAencoding a secretory leader is operably linked to DNA for a polypeptideif it is expressed as a preprotein that participates in the secretion ofthe polypeptide; a promoter or enhancer is operably linked to a codingsequence if it affects the transcription of the sequence; or a ribosomebinding site is operably linked to a coding sequence if it is positionedso as to facilitate translation. Generally, “operably linked” means thatthe DNA sequences being linked are contiguous, and, in the case of asecretory leader, contiguous and in reading phase. However, enhancers donot have to be contiguous. Linking is accomplished by ligation atconvenient restriction sites. If such sites do not exist, the syntheticoligonucleotide adaptors or linkers are used in accordance withconventional practice.

As used herein, the term “gene” means the segment of DNA involved inproducing a polypeptide chain, that may or may not include regionspreceding and following the coding region, e.g. 5′ untranslated (5′ UTR)or “leader” sequences and 3′ UTR or “trailer” sequences, as well asintervening sequences (introns) between individual coding segments(exons).

A nucleic acid sequence is considered to be “selectively hybridizable”to a reference nucleic acid sequence if the two sequences specificallyhybridize to one another under moderate to high stringency hybridizationand wash conditions. Hybridization conditions are based on the meltingtemperature (Tm) of the nucleic acid binding complex or probe. Forexample, “maximum stringency” typically occurs at about Tm-5° C. (5°below the Tm of the probe); “high stringency” at about 5-10° below theTm; “intermediate stringency” at about 10-20° below the Tm of the probe;and “low stringency” at about 20-250 below the Tm. Functionally, maximumstringency conditions may be used to identify sequences having strictidentity or near-strict identity with the hybridization probe; whilehigh stringency conditions are used to identify sequences having about80% or more sequence identity with the probe.

Moderate and high stringency hybridization conditions are well known inthe art (see, for example, Sambrook, et al, 1989, Chapters 9 and 11, andin Ausubel, F. M., et al., 1993, expressly incorporated by referenceherein). An example of high stringency conditions includes hybridizationat about 42° C. in 50% formamide, 5×SSC, 5×Denhardt's solution, 0.5% SDSand 100 μg/ml denatured carrier DNA followed by washing two times in2×SSC and 0.5% SDS at room temperature and two additional times in0.1×SSC and 0.5% SDS at 42° C.

As used herein, “recombinant” includes reference to a cell or vector,that has been modified by the introduction of a heterologous nucleicacid sequence or that the cell is derived from a cell so modified. Thus,for example, recombinant cells express genes that are not found inidentical form within the native (non-recombinant) form of the cell orexpress native genes that are otherwise abnormally expressed, underexpressed or not expressed at all as a result of deliberate humanintervention.

As used herein, the terms “transformed”, “stably transformed” or“transgenic” with reference to a cell means the cell has a non-native(heterologous) nucleic acid sequence integrated into its genome or as anepisomal plasmid that is maintained through two or more generations.

As used herein, the term “expression” refers to the process by which apolypeptide is produced based on the nucleic acid sequence of a gene.The process includes both transcription and translation.

The term “introduced” in the context of inserting a nucleic acidsequence into a cell, means “transfection”, or “transformation” or“transduction” and includes reference to the incorporation of a nucleicacid sequence into a eukaryotic or prokaryotic cell where the nucleicacid sequence may be incorporated into the genome of the cell (forexample, chromosome, plasmid, plastid, or mitochondrial DNA), convertedinto an autonomous replicon, or transiently expressed (for example,transfected mRNA).

The present invention provides novel gram-positive microorganismsecretion factors and methods that can be used in microorganisms toameliorate the bottleneck to protein secretion and the production ofproteins in secreted form, in particular when the proteins arerecombinantly introduced and overexpressed by the host cell. The presentinvention provides the secretion factors TatC and TatA derived fromBacillus subtilis. In particular, the TatCd and TatCy peptide, as wellas the genes encoding them, are described herein.

The recent discovery of a ubiquitous translocation pathway, specificallyrequired for proteins with a twin-arginine motif in their signalpeptide, has focused interest on its membrane-bound components, one ofwhich is known as TatC. Unlike most organisms of which the genome hasbeen sequenced completely, the Gram-positive eubacterium Bacillussubtilis contains two tatC-like genes, denoted tatCd and tatCy. Thecorresponding TatCd and TatCy proteins have the potential to be involvedin the translocation of 27 proteins with putative twin-arginine signalpeptides of which about 6 to 14 are likely to be secreted into thegrowth medium. Using a proteomic approach, we show that PhoD of B.subtilis, a phosphodiesterase belonging to a novel protein family ofwhich all known members are synthesized with typical twin-argininesignal peptides, is secreted via the twin-arginine translocationpathway. Strikingly, TatCd is of major importance for the secretion ofPhoD, whereas TatCy is not required for this process. Thus, TatC appearsto be a specificity determinant for protein secretion via the Tatpathway. Based on our observations, we hypothesize that theTatC-determined pathway specificity is based on specific interactionsbetween TatC-like proteins and other pathway components, such as TatA,of which three paralogues are present in B. subtilis.

Tat Nucleic Acid and Amino Acid Sequences

The TatCd polynucleotide having the sequence corresponding to the aminoacid sequence as shown in FIG. 1 or 14 encodes the Bacillus subtilissecretion factor TatCd. The Bacillus subtilis TatCd was identified via aFASTA search of Bacillus subtilis translated genomic sequences using aconsensus sequence of TatC derived from E. coli. A FASTA search ofBacillus subtilis translated genomic sequences with the E. coli TatCsequence alone did not identify the B. subtilis TatCd. The presentinvention provides gram-positive tatCd polynucleotides which may be usedalone or together with other secretion factors in a gram-positive hostcell for the purpose of increasing the secretion of desired heterologousor homologous proteins or polypeptides.

The present invention encompasses tatCd polynucleotide homologs encodingnovel gram-positive microorganism tatC whether encoded by one ormultiple polynucleotides which have at least 80%, or at least 90% or atleast 95% identity to B. subtilis TatCd as long as the homolog encodes aprotein that is able to function by modulating secretion in agram-positive microorganism. As will be understood by the skilledartisan, due to the degeneracy of the genetic code, a variety ofpolynucleotides, i.e., tatC polynucleotide variants, can encode theBacillus subtilis secretion factors TatCd. The present inventionencompasses all such polynucleotides.

The present invention encompasses novel tatCd polynucleotide homologsencoding gram-positive microorganism TatC which has at least 80%, or atleast 90% or at least 95% identity to B. subtilis as long as the homologencodes a protein that has activity in a secretion.

Gram-positive polynucleotide homologs of B. subtilis tatCd may beobtained by standard procedures known in the art from, for example,cloned DNA (e.g., a DNA “library”), genomic DNA libraries, by chemicalsynthesis once identified, by cDNA cloning, or by the cloning of genomicDNA, or fragments thereof, purified from a desired cell. (See, forexample, Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual,2d Ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.;Glover, D. M., (ed.), 1985, DNA Cloning: A Practical Approach, MRLPress, Ltd., Oxford, U.K. Vol. I, II.) A preferred source is fromgenomic DNA. Nucleic acid sequences derived from genomic DNA may containregulatory regions in addition to coding regions. Whatever the source,the isolated TatCd gene should be molecularly cloned into a suitablevector for propagation of the gene.

In the molecular cloning of the gene from genomic DNA, DNA fragments aregenerated, some of which will encode the desired gene. The DNA may becleaved at specific sites using various restriction enzymes.Alternatively, one may use DNAse in the presence of manganese tofragment the DNA, or the DNA can be physically sheared, as for example,by sonication. The linear DNA fragments can then be separated accordingto size by standard techniques, including but not limited to, agaroseand polyacrylamide gel electrophoresis and column chromatography.

Once the DNA fragments are generated, identification of the specific DNAfragment containing the tatCd may be accomplished in a number of ways.For example, a B. subtilis tatCd gene of the present invention or itsspecific RNA, or a fragment thereof, such as a probe or primer, may beisolated and labeled and then used in hybridization assays to detect agram-positive tatC gene. (Benton, W. and Davis, R., 1977, Science196:180; Grunstein, M. And Hogness, D., 1975, Proc. Natl. Acad. Sci. USA72:3961). Those DNA fragments sharing substantial sequence similarity tothe probe will hybridize under stringent conditions.

Accordingly, the present invention provides a method for the detectionof gram-positive TatCd polynucleotide homologs which compriseshybridizing part or all of a nucleic acid sequence of B. subtilis tatCdwith gram-positive microorganism nucleic acid of either genomic or cDNAorigin.

Also included within the scope of the present invention aregram-positive microorganism polynucleotide sequences that are capable ofhybridizing to the nucleotide sequence of B. subtilis tatCd underconditions of intermediate to maximal stringency. Hybridizationconditions are based on the melting temperature (Tm) of the nucleic acidbinding complex, as taught in Berger and Kimmel (1987, Guide toMolecular Cloning Techniques, Methods in Enzymology, Vol 152, AcademicPress, San Diego Calif.) incorporated herein by reference, and confer adefined “stringency” as explained below.

Also included within the scope of the present invention are novelgram-positive microorganism tatC polynucleotide sequences that arecapable of hybridizing to part or all of the tatC nucleotide sequence ofFigure ? under conditions of intermediate to maximal stringency.Hybridization conditions are based on the melting temperature (Tm) ofthe nucleic acid binding complex, as taught in Berger and Kimmel (1987,Guide to Molecular Cloning Techniques, Methods in Enzymology, Vol 152,Academic Press, San Diego Calif.) incorporated herein by reference, andconfer a defined “stringency” as explained below.

“Maximum stringency” typically occurs at about Tm−5° C. (5° C. below theTm of the probe); “high stringency” at about 5° C. to 10° C. below Tm;“intermediate stringency” at about 10° C. to 20° C. below Tm; and “lowstringency” at about 20° C. to 25° C. below Tm. As will be understood bythose of skill in the art, a maximum stringency hybridization can beused to identify or detect identical polynucleotide sequences while anintermediate or low stringency hybridization can be used to identify ordetect polynucleotide sequence homologs.

The term “hybridization” as used herein shall include “the process bywhich a strand of nucleic acid joins with a complementary strand throughbase pairing” (Coombs J (1994) Dictionary of Biotechnology, StocktonPress, New York N.Y.).

The process of amplification as carried out in polymerase chain reaction(PCR) technologies is described in Dieffenbach C W and G S Dveksler(1995, PCR Primer, a Laboratory Manual, Cold Spring Harbor Press,Plainview N.Y.). A nucleic acid sequence of at least about 10nucleotides and as many as about 60 nucleotides from the TatCdnucleotide sequence of Figure ?, preferably about 12 to 30 nucleotides,and more preferably about 20-25 nucleotides can be used as a probe orPCR primer.

The B. subtilis tatCd polynucleotide corresponding to the amino acidsequence as shown in FIG. 1 or 14 encodes B. subtilis TatCd. The presentinvention encompasses novel gram positive microorganism amino acidvariants of the amino acid sequence shown in FIG. 1 or 14 that are atleast 80% identical, at least 90% identical and at least 95% identicalto the sequence shown in FIG. 1 or 14 as long as the amino acid sequencevariant is able to function by modulating secretion of proteins ingram-positive microorganisms.

The secretion factor TatCd as shown in FIG. 1 was subjected to a FASTA(Lipmann Pearson routine) amino acid search against a consensus aminoacid sequence for TatCd. The amino acid alignment is shown in FIG. 1.

Expression Systems

The present invention provides expression systems for the enhancedproduction and secretion of desired heterologous or homologous proteinsin a host microorganism.

I. Coding Sequences

In the present invention, the vector comprises at least one copy ofnucleic acid encoding a gram-positive microorganism TatC and/or TatAsecretion factor and preferably comprises multiple copies. In apreferred embodiment, the gram-positive microorganism is Bacillus. Inanother preferred embodiment, the gram-positive microorganism isBacillus subtilis. In a preferred embodiment, polynucleotides whichencode B. subtilis TatC and/or TatA, or fragments thereof, or fusionproteins or polynucleotide homolog sequences that encode amino acidvariants of TatC and/or TatA, may be used to generate recombinant DNAmolecules that direct the expression of TatC and/or TatA, or amino acidvariants thereof, respectively, in gram-positive host cells. In apreferred embodiment, the host cell belongs to the genus Bacillus. Inanother preferred embodiment, the host cell is B. subtilis.

As will be understood by those of skill in the art, it may beadvantageous to produce polynucleotide sequences possessingnon-naturally occurring codons. Codons preferred by a particulargram-positive host cell (Murray E et al (1989) Nuc Acids Res 17:477-508)can be selected, for example, to increase the rate of expression or toproduce recombinant RNA transcripts having desirable properties, such asa longer half-life, than transcripts produced from naturally occurringsequence.

Altered gram positive tatC and/or tatA polynucleotide sequences whichmay be used in accordance with the invention include deletions,insertions or substitutions of different nucleotide residues resultingin a polynucleotide that encodes the same or a functionally equivalentTatC and/or TatA homolog, respectively. As used herein a “deletion” isdefined as a change in either nucleotide or amino acid sequence in whichone or more nucleotides or amino acid residues, respectively, areabsent.

As used herein an “insertion” or “addition” is that change in anucleotide or amino acid sequence which has resulted in the addition ofone or more nucleotides or amino acid residues, respectively, ascompared to the naturally occurring gram positive TatC and/or TatA.

As used herein “substitution” results from the replacement of one ormore nucleotides or amino acids by different nucleotides or amino acids,respectively.

The encoded protein may also show deletions, insertions or substitutionsof amino acid residues which produce a silent change and result in afunctionally equivalent gram-positive TatC and/or TatA variant.Deliberate amino acid substitutions may be made on the basis ofsimilarity in polarity, charge, solubility, hydrophobicity,hydrophilicity, and/or the amphipathic nature of the residues as long asthe variant retains the ability to modulate secretion. For example,negatively charged amino acids include aspartic acid and glutamic acid;positively charged amino acids include lysine and arginine; and aminoacids with uncharged polar head groups having similar hydrophilicityvalues include leucine, isoleucine, valine; glycine, alanine;asparagine, glutamine; serine, threonine, phenylalanine, and tyrosine.

The TatC and/or TatA polynucleotides of the present invention may beengineered in order to modify the cloning, processing and/or expressionof the gene product. For example, mutations may be introduced usingtechniques which are well known in the art, e.g., site-directedmutagenesis to insert new restriction sites, to alter glycosylationpatterns or to change codon preference, for example.

In one embodiment of the present invention, a TatC and/or TatApolynucleotide may be ligated to a heterologous sequence to encode afusion protein. A fusion protein may also be engineered to contain acleavage site located between the TatC and/or TatA nucleotide sequenceand the heterologous protein sequence, so that the TatC and/or TatAprotein may be cleaved and purified away from the heterologous moiety.

II. Vector Sequences

Expression vectors used in expressing the secretion factors of thepresent invention in gram-positive microorganisms comprise at least onepromoter associated with a gram-positive tatC and/or tatA, whichpromoter is functional in the host cell. In one embodiment of thepresent invention, the promoter is the wild-type promoter for theselected secretion factor and in another embodiment of the presentinvention, the promoter is heterologous to the secretion factor, butstill functional in the host cell.

Additional promoters associated with heterologous nucleic acid encodingdesired proteins or polypeptides may be introduced via recombinant DNAtechniques. In one embodiment of the present invention, the host cell iscapable of overexpressing a heterologous protein or polypeptide andnucleic acid encoding one or more secretion factor(s) is(are)recombinantly introduced. In one preferred embodiment of the presentinvention, nucleic acid encoding TatC and/or TatA is stably integratedinto the microorganism genome. In another embodiment, the host cell isengineered to overexpress a secretion factor of the present inventionand nucleic acid encoding the heterologous protein or polypeptide isintroduced via recombinant DNA techniques. The present inventionencompasses gram-positive host cells that are capable of overexpressingother secretion factors known to those of skill in the art, or othersecretion factors known to those of skill in the art or identified inthe future.

In a preferred embodiment, the expression vector contains a multiplecloning site cassette which preferably comprises at least onerestriction endonuclease site unique to the vector, to facilitate easeof nucleic acid manipulation. In a preferred embodiment, the vector alsocomprises one or more selectable markers. As used herein, the termselectable marker refers to a gene capable of expression in thegram-positive host which allows for ease of selection of those hostscontaining the vector. Examples of such selectable markers include butare not limited to antibiotics, such as, erythromycin, actinomycin,chloramphenicol and tetracycline.

III. Transformation

In one embodiment of the present invention, nucleic acid encoding one ormore gram-positive secretion factor(s) of the present invention isintroduced into a gram-positive host cell via an expression vectorcapable of replicating within the host cell. Suitable replicatingplasmids for Bacillus are described in Molecular Biological Methods forBacillus, Ed. Harwood and Cutting, John Wiley & Sons, 1990, herebyexpressly incorporated by reference; see chapter 3 on plasmids. Suitablereplicating plasmids for B. subtilis are listed on page 92.

In another embodiment, nucleic acid encoding a gram-positivemicro-organism tatC and/or tatA stably integrated into the microorganismgenome. Preferred gram-positive host cells are from the genus Bacillus.Another preferred gram-positive host cell is B. subtilis. Severalstrategies have been described in the literature for the direct cloningof DNA in Bacillus. Plasmid marker rescue transformation involves theuptake of a donor plasmid by competent cells carrying a partiallyhomologous resident plasmid (Contente et al., Plasmid 2:555-571 (1979);Haima et al., Mol. Gen. Genet. 223:185-191 (1990); Weinrauch et al., J.Bacteriol. 154(3):1077-1087 (1983); and Weinrauch et al., J. Bacteriol.169(3):1205-1211 (1987)). The incoming donor plasmid recombines with thehomologous region of the resident “helper” plasmid in a process thatmimics chromosomal transformation.

Transformation by protoplast transformation is described for B. subtilisin Chang and Cohen, (1979) Mol. Gen. Genet. 168:111-115; for B.megaterium in Vorobjeva et al., (1980) FEMS Microbiol. Letters7:261-263; for B. amyloliquefaciens in Smith et al., (1986) Appl. andEnv. Microbiol. 51:634; for B. thuringiensis in Fisher et al., (1981)Arch. Microbiol. 139:213-217; for B. sphaericus in McDonald (1984) J.Gen. Microbiol. 130:203; and B. larvae in Bakhiet et al., (1985) 49:577.Mann et al., (1986, Current Microbiol. 13:131-135) report ontransformation of Bacillus protoplasts and Holubova, (1985) FoliaMicrobiol. 30:97) disclose methods for introducing DNA into protoplastsusing DNA containing liposomes.

Identification of Transformants

Although the presence/absence of marker gene expression suggests thatthe gene of interest is also present, its presence and expression shouldbe confirmed. For example, if the nucleic acid encoding tatC and/or tatAis inserted within a marker gene sequence, recombinant cells containingthe insert can be identified by the absence of marker gene function.Alternatively, a marker gene can be placed in tandem with nucleic acidencoding the secretion factor under the control of a single promoter.Expression of the marker gene in response to induction or selectionusually indicates expression of the secretion factor as well.

Alternatively, host cells which contain the coding sequence for asecretion factor and express the protein may be identified by a varietyof procedures known to those of skill in the art. These proceduresinclude, but are not limited to, DNA-DNA or DNA-RNA hybridization andprotein bioassay or immunoassay techniques which include membrane-based,solution-based, or chip-based technologies for the detection and/orquantification of the nucleic acid or protein.

The presence of the tatC and/or tatA polynucleotide sequence can bedetected by DNA-DNA or DNA-RNA hybridization or amplification usingprobes, portions or fragments derived from the B. subtilis tatC and/ortatA polynucleotide.

Secretion Assays

Means for determining the levels of secretion of a heterologous orhomologous protein in a gram-positive host cell and detecting secretedproteins include, using either polyclonal or monoclonal antibodiesspecific for the protein. Examples include enzyme-linked immunosorbentassay (ELISA), radioimmunoassay (RIA) and fluorescent activated cellsorting (FACS). These and other assays are described, among otherplaces, in Hampton R et al (1990, Serological Methods, a LaboratoryManual, APS Press, St Paul Minn.) and Maddox D E et al (1983, J Exp Med158:1211).

A wide variety of labels and conjugation techniques are known by thoseskilled in the art and can be used in various nucleic and amino acidassays. Means for producing labeled hybridization or PCR probes fordetecting specific polynucleotide sequences include oligolabeling, nicktranslation, end-labeling or PCR amplification using a labelednucleotide. Alternatively, the nucleotide sequence, or any portion ofit, may be cloned into a vector for the production of an mRNA probe.Such vectors are known in the art, are commercially available, and maybe used to synthesize RNA probes in vitro by addition of an appropriateRNA polymerase such as T7, T3 or SP6 and labeled nucleotides.

A number of companies such as Pharmacia Biotech (Piscataway N.J.),Promega (Madison Wis.), and US Biochemical Corp (Cleveland Ohio) supplycommercial kits and protocols for these procedures. Suitable reportermolecules or labels include those radionuclides, enzymes, fluorescent,chemiluminescent, or chromogenic agents as well as substrates,cofactors, inhibitors, magnetic particles and the like. Patents teachingthe use of such labels include U.S. Pat. Nos. 3,817,837; 3,850,752;3,939,350; 3,996,345; 4,277,437; 4,275,149 and 4,366,241. Also,recombinant immunoglobulins may be produced as shown in U.S. Pat. No.4,816,567 and incorporated herein by reference.

Purification of Proteins

Host cells transformed with polynucleotide sequences encodingheterologous or homologous protein may be cultured under conditionssuitable for the expression and recovery of the encoded protein fromcell culture. The protein produced by a recombinant host cell comprisinga secretion factor of the present invention will be secreted into theculture media. Other recombinant constructions may join the heterologousor homologous polynucleotide sequences to nucleotide sequence encoding apolypeptide domain which will facilitate purification of solubleproteins (Kroll D J et al (1993) DNA Cell Biol 12:441-53).

Such purification facilitating domains include, but are not limited to,metal chelating peptides such as histidine-tryptophan modules that allowpurification on immobilized metals (Porath J (1992) Protein Expr Purif3:263-281), protein A domains that allow purification on immobilizedimmunoglobulin, and the domain utilized in the FLAGS extension/affinitypurification system (Immunex Corp, Seattle Wash.). The inclusion of acleavable linker sequence such as Factor XA or enterokinase (Invitrogen,San Diego Calif.) between the purification domain and the heterologousprotein can be used to facilitate purification.

In the present studies, we demonstrate for the first time that afunctional Tat pathway, required for secretion of the PhoD protein,exists in the Gram-positive eubacterium B. subtilis. The TatCd protein,specified by one of the two tatC genes of B. subtilis, plays a criticalrole in this secretion pathway. In contrast, the TatCy protein appearsto be of minor importance for PhoD secretion. Even though no particularfunction for TatCy was identified, our results show that thecorresponding gene is transcribed under conditions of phosphatestarvation when TatCd fulfils its critical role in PhoD secretion.Furthermore, as inferred from the fact that low levels of PhoD secretionby B. subtilisΔtatCd (but never by tatCd-tatCy double mutants) wereobserved in some experiments, TatCy seems to be actively involved inRR-pre-protein translocation. Notably, these observations imply thatTatC is a specificity determinant for protein secretion via the Tatpathway. In fact, our observation that the secretion of PhoD wasincreased in the absence of TatCy suggests that abortive interactionsbetween pre-PhoD and TatCy or TatCy-containing translocases can occur.Nevertheless, alternative, more indirect explanations for thisobservation can presently not be excluded. Interestingly, the positiveeffect of the tatCy mutation on PhoD secretion is reminiscent of theeffect that was observed when certain genes (i.e. sipS and/or sipU) forparalogous type I signal peptidases of B. subtilis were disrupted. Thisresulted in significantly improved rates of processing of the α-amylaseAmyQ precursor by the remaining type I signal peptidases (i.e. SipT,SipV and/or SipW; Tjalsma et al. (1998) Genes Dev. 12, 2318-2331,Tjalsma et al. (1997) J. Biol. Chem. 272, 25983-25992, and Bolhuis etal. (1996). Mol. Microbiol. 22, 605-618). Taken together, theseobservations suggest that, in general, the presence of two or moreparalogous secretion machinery components in B. subtilis may result in,as yet undefined, abortive interactions with certain secretorypre-proteins.

The PhoD protein of B. subtilis is synthesized with a typical RR-signalpeptide that contains a long hydrophilic N-region with a consensusRR-motif, and a mildly hydrophobic H-region (Table I). In fact, theRR-signal peptide of PhoD contains no detectably atypical features forRR-signal peptides (see: Berks, B. C. (1996) Mol. Microbiol. 22,393-404) and, therefore, it is presently not clear why PhoD specificallyrequires the presence of TatCd for efficient secretion. Strikingly, thesecretion of YdhF, the only other protein with a predicted RR-signalpeptide that could, so far, be identified through 2D-gelelectrophoresis, was not affected in the ΔtatCd-ΔtatCy mutant. Thisobservation shows that the RR-motif in the YdhF signal peptide does notdirect this protein into the Tat pathway. Instead, YdhF is, most likely,secreted via the Sec pathway, which could be due to the relativelyshort, but highly hydrophobic, H-region of the YdhF signal peptide.Similarly, the WapA and WprA proteins of B. subtilis, which havepredicted RR-signal peptides (Table I), were recently shown to besecreted in a strongly Ffh- and SecA-dependent manner (Hirose et al.(2000) Microbiology 146, 65-75), which implies that these proteins donot use the Tat pathway. Even though the H-regions of these signalpeptides are of similar size as that of the PhoD signal peptide, theyare significantly more hydrophobic. The latter observation suggeststhat, like in E. coli (Cristóbal et al. (1999) EMBO J. 18, 2982-2990),the hydrophobicity of the H-region is an important determinant thatallows the cell to discriminate between Sec-type and RR-signal peptides.Notably, the predicted RR-motifs of WapA, WprA and YdhF are alsodifferent from previously described RR-signal peptides, because theycontain Lys or Ser residues at the +3 position relative to thetwin-arginines (Table I). In fact, hydrophilic residues are completelyabsent from the +2 and +3 positions, relative to the twin-arginines ofknown RR-signal peptides (Berks, B. C. (1996) Mol. Microbiol. 22,393-404, Brink et al. (1998) FEBS Lett. 434, 425-430, Sargent et al.(1998) EMBO J. 17, 3640-3650, Chaddock et al. (1995) EMBO J. 14,2715-2722, Sargent et al. (1999) J. Biol. Chem. 274, 36073-36082, andSantini et al., (1998) EMBO J. 17, 101-112). If low overallhydrophobicity and the presence of hydrophobic residues at the +2 and +3positions are used as criteria for the prediction of RR-signal peptides,the total number of predicted B. subtilis signal peptides of this typecan be reduced from 27 to 11. Notably, of these 11 pre-proteins, 4contain additional transmembrane segments, and 1 lacks a signalpeptidase cleavage site. Thus, based on these more stringent criteria,one would predict that merely 6 proteins of B. subtilis (i.e. AlbB,LipA, PhoD, YkpC, YkuE, and YwbN) are secreted into the growth mediumvia the Tat pathway. This would explain why the secretion of only oneprotein, PhoD, was detectably affected in B. subtilis ΔtatCd-ΔtatCyunder conditions of phosphate starvation. In this respect, it isimportant to note that TatC-dependent secretion of some other proteinswith (predicted) RR-signal peptides may have remained unnoticed in thepresent studies, because they are expressed at very low levels underconditions of phosphate starvation. Furthermore, it is conceivable thatother TatC-dependent proteins were missed in the 2D-gel electrophoreticanalysis, due to their poor separation in the first dimension.

Interestingly, the YdhF protein was also predicted to be a lipoprotein(Table I; Tjalsma et al. (1999) J. Biol. Chem. 274, 1698-1707). The factthat YdhF was found in the growth medium either suggests that thisprediction was wrong, or that YdhF is released into the growth mediumvia a secondary processing event that follows cleavage by thelipoprotein-specific (type II) signal peptidase (Prágai et al. (1997)Microbiology 143, 1327-1333). Such secondary processing events have beendescribed previously for other Bacillus lipoproteins (see: Tjalsma etal. (1999) J. Biol. Chem. 274, 1698-1707). In fact, the latterpossibility most likely explains why the phosphate-binding protein PstS,which is a typical lipoprotein (previously known as YqgG; Tjalsma et al.(1999) J. Biol. Chem. 274, 1698-1707, and Qi, Y., and Hulett, F. M.(1998) J. Bacteriol. 180, 4007-4010), was found in the growth medium. Asexpected for lipoproteins, significant amounts of PstS were also presentin a cell-associated form (Antelmann, H., Scharf, C., and Hecker, M.,(2000) J. Bacteriol. in press, and Eymann et al. (1996) Microbiology142, 3163-3170).

One of the outstanding features of the Tat pathway of E. coli is itsability to translocate fully-folded proteins that bind cofactors priorto export from the cytoplasm, and even multimeric enzyme complexes(Berks, B. C. (1996) Mol. Microbiol. 22, 393-404, Weiner et al. (1998)Cell 93, 93-101, Santini et al. (1998) EMBO J. 17, 101-112, and Rodrigueet al. (1999) J. Biol. Chem. 274, 13223-13228). Similarly, thethylakoidal Tat pathway has been shown to translocate folded proteins(Bogsch et al. (1997) EMBO J. 16, 3851-3859, and Hynds et al. (1998) J.Biol. Chem. 273, 34868-34874). Thus, it seems as if this pathway is usedfor the transport of proteins that are Sec-incompatible, either becausethey must fold before translocation, or because they fold too rapidly ortightly to allow transport via the Sec-system, which is known totransport proteins in an unfolded conformation (see: Dalbey, R. E., andRobinson, C. (1999) Trends Biochem. Sci. 24, 17-22). Consistent withthis idea, folded pre-proteins, some of which were biologically active,were shown to accumulate in tat mutants of E. coli (Sargent et al.(1998) EMBO J. 17, 3640-3650, Bogsch et al. (1998) J. Biol. Chem. 273,18003-18006, Weiner et al. (1998) Cell 93, 93-101, and Sargent et al.(1999) J. Biol. Chem. 274, 36073-36082). Therefore, it is conceivablethat the Tat pathway of B. subtilis is also involved in the transport offolded cofactor-binding proteins. This view is supported by theobservation that the iron-sulfur cluster-binding Rieske protein QcrA ofB. subtilis (Yu et al. (1995) J. Bacteriol. 177, 6751-6760) issynthesised with a predicted RR-signal peptide (Table I). Nevertheless,compared to the parental strain, pre-PhoD accumulation was not increasedin B. subtilis ΔtatCd-ΔtatCy. This suggests that pre-PhoD is either notfolded prior to translocation, or that folded pre-PhoD is sensitive tocytosolic proteases of B. subtilis. We favor the first possibility,because most native B. subtilis proteins are highly resistant toproteolysis, provided that they are properly folded (see: Stephenson etal. (1998) Appl. Environ. Microbiol. 64, 2875-2881, Bolhuis et al.(1999) J. Biol. Chem. 274, 15865-15868, and Bolhuis et al. (1999) Appl.Environ. Microbiol. 65, 2934-2941). Consistent with the idea thatpre-PhoD could be secreted in a loosely folded or unfolded conformationis the observation that loosely folded proteins can be transported viathe thylakoidal Tat pathway (Bogsch et al. (1997) EMBO J. 16, 3851-3859,and Hynds et al. (1998) J. Biol. Chem. 273, 34868-34874). Strikingly,the four known homologues of PhoD, all of which were identified inStreptomyces species, are synthesised with a typical RR-signal peptide(Table IV). Thus it seems that PhoD-like proteins belong to a novelfamily of proteins with an as yet undefined requirement fortranslocation via the Tat pathway. In this respect, it is interesting tonote that the N-regions of the RR-signal peptides of PhoD and PhoD-likeproteins are among the longest N-regions of known RR-signal peptides(see: Berks, B. C. (1996) Mol. Microbiol. 22, 393-404).

Finally, one of the most striking results of our present studies is theobservation that TatC is a specificity determinant for protein secretionvia the Tat pathway of B. subtilis. Interestingly, this findingquestions to some extent the hypothesis that the TatA-like components ofthis pathway have a receptor-like function (Chanal et al. (1998) Mol.Microbiol. 30, 674-676, and Settles et al. (1997) Science 278,1467-1470). Instead, it suggests that TatC-like proteins recognisespecific elements of certain exported proteins, such as the RR-signalpeptide. Thus, our results might represent the first experimentalsupport for the ‘sea anemone’ model of Berks et. al. (Mol. Microbiol.(2000) 5, 260-274) in which, on the basis of theoretical considerations,it is proposed that the TatABE proteins form a protein-conductingchannel, while the TatC protein acts as an RR-signal peptide receptor.Alternatively, it is still conceivable that certain proteins withRR-signal peptides are recognized by TatA-like proteins, provided that aspecific TatC-like partner protein is present. A third possibility wouldbe that specific TatA- and TatC-like partner proteins are jointlyinvolved in substrate recognition. The fact that neither TatAc nor TatAdof B. subtilis were able to complement tatA, tatB or tatE mutations inE. coli, and that TatCd of B. subtilis was unable to complement the E.coli tatC mutation (our unpublished observations), suggests that theTatC-determined pathway specificity, as described in the presentstudies, is based on specific interactions between TatA- and TatC-likeproteins. If so, this implies that B. subtilis contains two parallelroutes for twin-arginine translocation, one of which involves the TatCdprotein. As shown in the present studies, the TatCd-dependenttranslocation appears to be activated specifically under conditions ofphosphate starvation, perhaps with the sole purpose of translocatingPhoD. Similar to the situation in B. subtilis, parallel routes fortwin-arginine translocation may be present in other organisms, such asArchaeoglobus fulgidus, which was shown to contain two paralogoustatC-like genes (Berks et al. (2000) Mol. Microbiol. 5, 260-274, andKlenk et al. (1997) Nature 390, 364-370).

Additional work carried out in support of the present inventionindicates that both tatCd and tatCy may be TAT components andresponsible for secretion of other genes as well. In fact, withreference to FIG. 6, a tatCd deletion totally abolishes the secretion ofLipA. FIG. 6 however suggests also that, while TatCd is the primary TATcomponent, TatCy plays some role on the secretion of LipA (although notas stringent as TatCd).

The bacterial twin-arginine translocation (Tat) pathway has beenrecently described for PhoD of Bacillus subtilis, a phosphodiesterasecontaining a twin-arginine signal peptide. The expression of phoD,induced in response to phosphate depletion, is co-regulated withexpression of tatA_(d) and tatC_(d) genes localized downstream of phoD.While tatC_(d) was of major importance for the secretion of PhoD, thesecond copy of a tatC (tatC_(y)) was not required for this process. Tocharacterise specificity of PhoD transport further, translocation ofPhoD was investigated in E. coli. Using gene fusions, we analysed theparticular role of the signal peptide and the mature region of PhoD incanalising the transport route. A hybrid protein consisting of thesignal peptide of TEM-β-lactainase and mature PhoD was transportedSec-dependent indicating that the mature part of PhoD does not containinformation canalising the selected translocation route. PrePhoD as wellas a fusion protein consisting of the signal peptide of PhoD (SP_(phoD))and β-galactosidase (LacZ) remained cytosolic in the Escherichia coli.Thus, SP_(phoD) appears to be not recognised by E. coli transportsystems. Co-expression of B. subtilis tatA_(d)/C_(d) genes resulted inthe processing of SP_(phoD)-LacZ and periplasmic localisation of LacZillustrating a close substrate-Tat component specificity of thePhoD-TatA_(d)/C_(d) transport system. While blockage of theSec-dependent transport did not affect the localisation ofSP_(phoD)-LacZ, translocation and processing was dependent on the pHgradient of the cytosolic membrane. TatAd/Cd-mediated transport ofSP_(phoD)-LacZ was observed in absence of the E. coli Tat proteinsindicating SP_(phoD)-peptides and its adopted TatAd/Cd protein pair forman autonomous Tat system in E. coli. Thus, the minimal requirement of anactive Tat-dependent protein translocation system consists of atwin-arginine signal peptide containing Tat substrate, its specificTatA/C proteins and the pH-gradient across the cytosolic membrane.

The following preparations and examples are given to enable thoseskilled in the art to more clearly understand and practice the presentinvention. They should not be considered as limiting the scope and/orspirit of the invention, but merely as being illustrative andrepresentative thereof.

In the experimental disclosure which follows, the followingabbreviations apply: eq (equivalents); M (Molar); μM (micromolar); N(Normal); mol (moles); mmol (millimoles); μmol (micromoles); nmol(nanomoles); g (grams); mg (milligrams); kg (kilograms); μg(micrograms); L (liters); ml (milliliters); μl (microliters); cm(centimeters); mm (millimeters); μm (micrometers); nm (nanometers); ° C.(degrees Centigrade); h (hours); min (minutes); sec (seconds); msec(milliseconds); TLC (thin layer achromatography); TY, trypton/yeastextract; Ap, ampicillin; DTT, dithiotreitol; Em, erythromycin; HPDM,high phosphate defined medium; IPG, immobilized pH gradient; IPTG,isopropyl-β-D-thiogalactopyranbside; Km, kanamycin; LPDM, low phosphatedefined medium; MM, minimal medium; OD, optical density; PAGE,polyacrylamide gel electrophoresis; PCR, polymerase chain reaction; Sp,spectinomycin; SSM, Schaeffer's sporulation medium; 2D, two-dimensional.

EXAMPLE 1 Identification of Tat Genes of B. subtilis

In order to investigate whether B. subtilis contains a potential Tatpathway, a search for homologues of E. coli Tat proteins was performed,using the complete sequence of the B. subtilis genome (Kunst et al.(1997) Nature 390, 249-256). First, sequence comparisons revealed thatB. subtilis contains three paralogous genes (i.e., yczB, ydiI and ynzA)that specify proteins with sequence similarity to the three paralogousE. coli TatA, TatB and TatE proteins. Specifically, the YdiI protein (57residues), which was renamed TatAy, showed the highest degree ofsequence similarity with the E. coli TatA protein (58% identicalresidues and conservative replacements); the YczB protein (70 residues),which was renamed TatAd, showed the highest degree of sequencesimilarity with the E. coli TatB protein (54% identical residues andconservative replacements); and the YnzA protein (62 residues), whichwas renamed TatAc, showed the highest degree of sequence similarity withthe E. coli TatB protein (53% identical residues and conservativereplacements). All three B. subtilis proteins were renamed TatA to avoidpossible mis-interpretations with respect to their respective functions,which are presently unknown. Like TatA, TatB, and TatE of E. coli, thethree TatA proteins of B. subtilis appear to have one amino-terminalmembrane spanning domain (FIG. 1A), and the carboxyl-terminal parts ofthese proteins are predicted to face the cytoplasm. Even though TatAc,TatAd and TatAy of B. subtilis show significant similarity to TatA, TatBand TatE of E. coli when the amino acid sequences of these proteins arecompared pairwise, only a limited number of residues is conserved in allsix amino acid sequences (17% identical residues and conservativereplacements; FIG. 1A).

Second, in contrast to E. coli, which contains a unique tatC gene (10),B. subtilis was shown to contain two paralogous tatC-like genes (ie.ycbT and ydiJ). The YcbT protein (245 residues), which was renamedTatCd, and the YdiJ protein (254 residues), which was renamed TatCy,showed significant similarity to the E. coli TatC protein (57% identicalresidues and conservative replacements in the three aligned sequences;FIG. 1B). Like TatC of E. coli, TatCd and TatCy of B. subtilis have sixpotential transmembrane segments (FIG. 1B), and the amino-termini ofthese proteins are predicted to face the cytoplasm (data not shown).

In contrast to E. coli, in which the tatA, tatB and tatC genes form oneoperon while the tatE gene is monocistronic (Sargent et al. (1998) EMBOJ. 17, 3640-3650), the tat genes of B. subtilis are located at threedistinct chromosomal regions. Two of these regions contain adjacent tatAand tatC genes, the tatAd and tatAy genes being located immediatelyupstream of the tatCd and tatCy genes, respectively (FIG. 2).Strikingly, the tatAd and tatCd genes, which map at 24.4 o on the B.subtilis chromosome, are located immediately downstream of the phoDgene, specifying a secreted protein with a putative RR-signal peptide(Table I). Furthermore, the tatAy and tatCy genes are located at 55.3°on the B. subtilis chromosome, within a cluster of genes with unknownfunction (FIG. 2), and the tatAc gene is located at 162.7° on the B.subtilis chromosome (data not shown), immediately downstream of the cotCgene specifying a spore coat protein (Donovan et al. (1987) J. Mol.Biol. 196, 1-10). Finally, a tatD-like gene, denoted yabD, is located at4.1° on the B. subtilis chromosome, immediately downstream of the metSgene encoding a methionyl-tRNA synthetase (data not shown).

Taken together, these observations strongly suggest that B. subtilis hasa Tat pathway for the translocation of proteins with RR-signal peptidesacross the cytoplasmic membrane. Furthermore, the observation that thetatAd and tatCd genes are located downstream of the phoD gene, which isa member of the pho regulon (Eder et al. (1996) Microbiology 142,2041-2047), suggests that the tatAd and tatCd genes might be exclusivelyexpressed under conditions of phosphate starvation.

EXAMPLE 2 TatC-dependent Secretion of the PhoD Protein

To investigate whether an active Tat pathway exists in B. subtilis,various single and double tatC mutants were constructed. To thispurpose, the tatCd gene was either disrupted with a Km resistancemarker, or it was placed under the control of the IPTG-dependent Pspacpromoter of plasmid pMutin2, resulting in the B. subtilis strains ΔtatCdand ItatCd, respectively (FIG. 3, A and B). Similarly, the tatCy genewas either disrupted with an Sp resistance marker, or it was placedunder the control of the IPTG-dependent Pspac promoter of plasmidpMutin2, resulting in the B. subtilis strains ΔtatCy and ItatCy,respectively (FIG. 3, A and C). Double tatCd-tatCy mutants wereconstructed by transforming the ΔtatCy mutant with chromosomal DNA ofthe ΔtatCd or ItatCd mutant strains.

Table II lists the plasmids and bacterial strains used. TY¹ medium(tryptone/yeast extract) contained Bacto tryptone (1%), Bacto yeastextract (0.5%) and NaCl (1%). Minimal medium (MM) was prepared asdescribed in Tjalsma et al. (1998) Genes Dev. 12, 2318-2331. Schaeffer'ssporulation medium (SSM) was prepared as described in Schaeffer et al.(1965) Proc. Natl. Acad. Sci. USA 271, 5463-5467. High phosphate (HPDM)and low phosphate (LPDM) defined media were prepared as described inMüller et al. (1997) Microbiology 143, 947-956. To test anaeorobicgrowth, S7 medium was prepared as described in van Diji et al. (1991) J.Gen. Microbiol. 137, 2073-2083 and van Diji et al. (1991) Mol. Gen.Genet. 227, 40-48 and supplemented with NaNO3 (0.2%) and glycerol (2%).When required, media for E. coli were supplemented with ampicillin (Ap;100 μg/ml), erythromycin (Em; 100 μg/ml), kanamycin (Km; 40 μg/ml), orspectinomycin (Sp; 100 μg/ml); media for B. subtilis were supplementedwith Em (1 μg/ml), Km (10 μg/ml), Sp (100 μg/ml), and/orisopropyl-β-D-thiogalactopyranoside (IPTG; 100 μM).

Procedures for DNA purification, restriction, ligation, agarose gelelectrophoresis, and transformation of E. coli were carried out asdescribed in Sambrook et al. (1989) Molecular Cloning: A laboratoryManual, 2nd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.Enzymes were from Roche Molecular Biochemicals. B. subtilis wastransformed as described in Tjalsma et al. (1997) J. Biol. Chem. 272,25983-25992. PCR (polymerase chain reaction) was carried out with thePwo DNA polymerase (New England Biolabs) as described in van Dijl et al.(1995) J. Biol. Chem. 270, 3611-3618.

To construct B. subtilis ItatCd, the 5′ region of the tatCd gene wasamplified by PCR with the primers JJ14bT (5′-CCC AAG CTT ATG AAA GGG AGGGCT TTT TTG AAT GG-3′) containing a HindIII site, and JJ15bT (5′-GCG GATCCA AAG CTG AGC ACG ATC GG-3′) containing a BamHI site. The amplifiedfragment was cleaved with HindIII and BamHI, and cloned in thecorresponding sites of pMutin2 (Vagner et al. (1998) Microbiol. 144,3097-3104), resulting in pMICd1. B. subtilis ItatCd was obtained by aCampbell-type integration (single cross-over) of pMICd1 into the tatCdregion of the chromosome.

To construct B. subtilis ItatCy, the 5′ region of the tatCy gene wasamplified by PCR with the primers JJ03iJ (5′-CCC AAG CTT AAA AAG AAA GAAGAT CAG TAA GTT AGG ATG-3′) containing a HindIII site, and JJ04iJ(5′-GCG GAT CCA AGT CCT GAG AAA TCC G-3′) containing a BamHI site. Theamplified fragment was cleaved with HindIII and BamHI, and cloned in thecorresponding sites of pMutin2, resulting in pMICy1. B. subtilis ItatCywas obtained by a Campbell-type integration (single crossover) of pMICy1into the tatCy region of the chromosome.

To construct B. subtilis ΔtatCd, the tatCd gene was amplified by PCRwith primer JJ33Cdd (5′-GGA ATT CGT GGG ACG GCT ACC-3′) containing anEcoRI site and 5′ sequences of tatCd, and primer JJ34Cdd (5′-CGG GAT CCATCA TGG GAA GCG-3′) containing a BamHI site and 3′ sequences of tatCd.Next, the PCR-amplified fragment was cleaved with EcoRI and BamHI andligated into the corresponding sites of pUC21, resulting in pJCd1.Plasmid pJCd2 was obtained by replacing an internal Bcll-AccI fragmentof the tatCd gene in pJCd1 with a pDG792-derived Km resistance marker,flanked by BamHI and ClaI restriction sites. Finally, B. subtilis ΔtatCdwas obtained by a double cross-over recombination event between thedisrupted tatCd gene of pJCd2 and the chromosomal tatCd gene.

To construct B. subtilis ΔtatCy, the tatCy gene was amplified by PCRwith primer JJ29Cyd (5′-GGG GTA CCG GAA AAC GCT TGA TCA GG-3′)containing a KpnI site and 5′ sequences of tatCy, and primer JJ30Cyd(5′-CGG GAT CCT TTG GGC GAT AGC C-3′) containing a BamHI site and 3′sequences of tatCy. Next, the PCR-amplified fragment was cleaved withKpnI and BamHI and ligated into the Asp718 and BamHI sites of pUC21,resulting in pJCy1. Plasmid pJCy2 was obtained by ligating apDG1726-derived Sp resistance marker, flanked by PstI restriction sites,into the unique PstI site of the tatCy gene in pJCy1. Finally, B.subtilis ΔtatCy was obtained by a double cross-over recombination eventbetween the disrupted tatCy gene of pJCy2 and the chromosomal tatCygene.

Double tatCd-tatCy mutants were constructed by transforming the ΔtatCymutant with chromosomal DNA of the ΔtatCd or ItatCd mutant strains.Correct integration of plasmids or resistance markers into thechromosome of B. subtilis was verified by Southern blotting. The BLASTalgorithm (Altschul et al. (1997) Nucleic Acids Res. 25, 3389-3402) wasused for protein comparisons in GenBank. Protein sequence alignmentswere carried out with the ClustalW program (Thompson et al. (1994)Nucleic Acids Res. 22, 4673-4680), using the Blosum matrices, or version6.7 of the PCGene Analysis Program (Intelligenetics Inc.). Putativetransmembrane segments, and their membrane topologies were predictedwith the TopPred2 algorithm (Sipos et al. (1993) Eur. J. Biochem. 213,1333-1340 and Cserzo et al. (1997) Protein Eng. 10, 673-676).

Competence and sporulation—Competence for DNA binding and uptake wasdetermined by transformation with plasmid or chromosomal DNA (Bron etal. (1972) Mutat. Res. 15, 1-10). The efficiency of sporulation wasdetermined by overnight growth in SSM medium, killing of cells with 0.1volume chloroform, and subsequent plating.

Western blot analysis and immunodetection—To detect PhoB and PhoD, B.subtilis cells were separated from the growth medium by centrifugation(2 min, 14.000 rpm, room temperature). Proteins in the growth mediumwere concentrated 20-fold upon precipitation with trichloroacetic acid,and samples for SDS polyacrylamide gel electrophoresis (PAGE) wereprepared as described previously in Laemmli, U. K. (1970) Nature 227,680-685. After separation by SDS-PAGE, proteins were transferred to anitrocellulose membrane (Schleicher and Schüll) as described in Towbinet al. (1979) Proc. Natl. Acad. Sci. USA 76, 4350-4354. PhoB and PhoDwere visualized with specific antibodies (Müller, J. P., and Wagner, M.(1999) FEMS Microbiol. Lett. 180, 287-296) and alkalinephosphatase-conjugated goat anti-rabbit antibodies (SIGMA) according tothe manufacturer's instructions.

Two-dimensional (2D) gel electrophoresis of secreted proteins. B.subtilis strains were grown at 37° C. under vigorous agitation in 1liter of a synthetic medium (Antelmann et al. (1997) J. Bacteriol. 179,7251-7256, and Antelmann et al., (2000) J. Bacteriol. in press)containing 0.16 mM KH₂PO₄ to induce a phosphate starvation response.After 1 hour of post-exponential growth, cells were separated from thegrowth medium by centrifugation. The secreted proteins in the growthmedium were precipitated overnight with ice-cold 10% trichloroaceticacid, and collected by centrifugation (40000 g, 2 h, 4° C.). The pelletwas washed 3 times with 96% ethanol, dried and resuspended in 400 μl ofrehydration solution containing 2 M thiourea, 8 M urea, 1% Nonidet P40,20 mM DTT and 0.5% Pharmalyte (pH 3-10). Cells were disrupted bysonication as described in Eymann et al. (1996) Microbiology 142,3163-3170, and cellular proteins were resuspended in rehydrationsolution as described above. Samples of secreted or cellular proteins inrehydration solution were used for the re-swelling of immobilized pHgradient (IPG) strips (pH range 3-10). Next, protein separation in theIPG strips (first dimension electrophoresis) was performed asrecommended by the manufacturer (Amersham Pharmacia Biotech).Electrophoresis in the second dimension was performed as described inBernhardt et al. (1997) Microbiology 143, 999-1017. The resulting2D-gels were stained with silver nitrate (Blum et al. (1987)Electrophoresis 8, 93-99) or Coomassie Brilliant Blue R250.

Protein identification. In-gel tryptic digestion of proteins, separatedby 2D gel electrophoresis, was performed using a peptide-collectingdevice (Otto et al. (1996) Electrophoresis 17, 1643-1650). To thispurpose, 0.5 μl peptide solution was mixed with an equal volume of asaturated a-cyano-4-hydroxy cinnamic acid solution in 50% acetonitrileand 0.1% trifluoroacetic acid. The resulting mixture was applied to thesample template of a matrix-assisted laser desorption/ionization massspectrometer (Voyager DE-STR, PerSeptive Biosystems). Peptide massfingerprints were analysed using the ‘MS-it’ software, as provided byBaker and Clausner through http://prospector.ucsf.edu.

The fact that double tatC-tatCy mutants could be obtained shows thatTatC function is not essential for viability of B. subtilis, at leastnot when cells are grown aerobically in TY or minimal medium at 37° C.,or anaerobically in S7 medium, supplemented with NaNO3 (0.2%) andglycerol (2%) at 37° C. (data not shown). Furthermore, the ΔtatCd-ΔtatCydouble mutation did not inhibit the development of competence for DNAbinding and uptake, sporulation and the subsequent spore germination(data not shown), showing that these primitive developmental processesdo not require TatC function.

The effects of single and double tatC mutations on protein secretion viathe Tat pathway were studied using PhoD as a native reporter protein. Tothis purpose, tatC mutant strains were grown under conditions ofphosphate starvation, using LPDM medium. As shown by Western blotting,the secretion of PhoD was strongly reduced in the ΔtatCd mutant strainand the ΔtatCd-ΔtatCy double mutant, whereas it was not affected or evenimproved in the ΔtatCy mutant strain (FIG. 4A). In contrast, thesecretion of the alkaline phosphatase PhoB, which is dependent of themajor (Sec) pathway for protein secretion (49), was not affected in thetatC mutants of B. subtilis (FIG. 4B). Notably, in some experiments,very low amounts of PhoD were detectable in the growth medium of B.subtilis ΔtatCd (data not shown), but never in that of ΔtatCd-ΔtatCy orItatCd-ΔtatCy double mutants (FIG. 4, A and C). As exemplified with theB. subtilis ItatCd-ΔtatCy double mutant strain, the cells of all tatCmutant strains contained similar amounts of pre-PhoD, which werecomparable to those in the parental strain 168 (FIG. 4C; data notshown). Finally, 2D-gel electrophoresis of proteins in the medium ofphosphate-starved cells of B. subtilis ΔtatCd-ΔtatCy or the parentalstrain 168 showed that PhoD is the only protein of which the secretionis detectably affected by the double tatC mutation under conditions ofphosphate starvation (FIG. 5). As expected, the secretion of proteinslacking an RR-signal peptide, such as the glycerophosphoryl diesterphosphodiesterase GlpQ, the pectate lyase Pel, the alkaline phosphatasesPhoA and PhoB, the phosphate-binding protein PstS, the minorextracellular serine protease Vpr, the PBSX prophage protein XkdE andthe protein with unknown function YncM, was not significantly affectedby the double tatC mutation. Surprisingly, however, the secretion of theYdhF, a protein of unknown function, which does have a potentialRR-signal peptide (Table I), was also not affected by the disruption oftatCd and tatCy (FIG. 5). Consistent with the above observations, nodifferences in the cellular proteomes of B. subtilis ΔtatCd-ΔtatCy andthe parental strain 168 could be detected by 2D-gel electrophoresis(data not shown).

In summary, these results show that an active Tat pathway exists in B.subtilis, and that TatCd has a critical role in the secretion of PhoD.

EXAMPLE 3 Expression of tatCd and tatCy Genes

To study the expression of the tatCd and tatCy genes, thetranscriptional tatCd-lacZ and tatCy-lacZ gene fusions, present in B.subtilis ItatCd and ItatCy, respectively, were used.

Enzyme activity assays—The assay and the calculation of β-galactosidaseunits (expressed as units per OD600) were carried out as described inMiller, J. H. (1982) Experiments in Molecular Biology, Cold SpringHarbor Laboratory Press, Cold Spring Harbor N.Y. Overnight cultures werediluted 100-fold in fresh medium and samples were taken at hourlyintervals for OD600 readings and β-galactosidase activitydeterminations. Induction of the phosphate starvation response wasmonitored by alkaline phosphatase activity determinations as describedin Hulett et al. (1990) J. Bacteriol. 172, 735-740.

As expected, upon a medium shift from high phosphate (HPDM) to lowphosphate (LPDM) medium in order to induce a phosphate starvationresponse, tatCd transcription could be observed in B. subtilis ItatCd.In this strain, relatively low, but constant levels of β-galactosidaseproduction were reached within a period of four hours after the changeto LPDM medium, while no β-galactosidase production was detectable inthe parental strain 168 (no lacZ gene fusion present; Table II). Incontrast, when cells of B. subtilis ItatCd were grown in minimal (MM),sporulation (SSM) or trypton/yeast extract (TY) media, none of whichinduces a phosphate starvation response, no transcription of the tatCdgene was detectable; under these conditions, the β-galactosidase levelsin cells of B. subtilis ItatCd were similar to those of the parentalstrain 168. Completely different results were obtained with B. subtilisItatCy: the tatCy gene was transcribed in all growth media tested and,notably, the transcription of tatCy in LPDM medium was much higher thanthat of the tatCd gene (Table III). In contrast to the tatCd gene, thehighest levels of tatCy transcription were observed in MM and TY medium,while the lowest levels of tatCy transcription were observed in SSMmedium (Table III). In conclusion, these findings show that tatCd isonly transcribed under conditions of phosphate starvation, in contrastto tatCy, which is transcribed under all conditions tested.

EXAMPLE 4 PhoD is not Transported in E. coli

Plasmids, bacterial strains and media—Table 5 lists the plasmids andbacterial strains used. TY medium (h-yptone/yeast extract) containedBacto wiptone (1%), Bacto yeast extract (0.5%) and NaCl (1%). Forpulse-chase labelling experiments M9-Minimal medium was prepared asdescribed (Miller et al. (1992) Suppression of the growth and exportdefects of an Escherichia coli secA(Ts) mutant by a gene cloned fromBacillus subtilis. Mol. Gen. Genet. 235, 89-96). When required, mediawere supplemented with ampicillin (100 μg/ml), kanamycin (40 μg/ml),chloramphenicol (20 μg/ml), tetracycline (12.5 μg/ml), arabinose (0.2%),isopropyl-β-D-thiogalactopyranoside (IPTG; 100 μM), nigericin (1 μM)and/or sodium azide (3 mM). [35S]-Methionine was from Hartman Analytic(Braunschweig, Germany), [14C]-labelled molecular weight marker fromAmersham International (Amersham, Bucks, U.K.).

DNA techniques—Procedures for DNA purification, restriction, ligation,agarose gel electrophoresis, and transformation of E. coli were carriedout as described in Sambrook et al. Restriction enzymes were from MBIFermentas. PCR (polymerase chain reaction) was carried out with the VENTDNA polymerase (New England Biolabs).

To construct pAR3phoD, the phoD gene including its ribosome binding sitewas amplified from the chromosome of B. subtilis strain 168 by PCR usingthe primers P1 (5′-GAG GAT CCA TGA GGA GAG AGG GGA TCT TGA ATG GCA TACGAC-3′) containing a BamHI site, and P2 (5′-CGA TCC TGC AGG ACC TCA TCGGAT TGC-3′) containing a PstI site. The amplified fragment was cleavedwith BamHI and PstI, and cloned in the corresponding sites of pAR3. Theresulting plasmid pAR3phoD allowed the arabinose inducible expression ofwild type phoD in E. coli.

To construct a gene fusion between bla and phoD genes, the signalsequence less phoD was amplified using primers P3 (5′-GTA GGA TCC GCGCCT AAC TTC TCA AGC-3′) containing a BamHI site and primer P2 containinga PstI site. The amplified fragment was cleaved with BamHI and PstI, andcloned in the corresponding sites of pUC19, resulting in plasmidpUC19′phoD. Next, the 5′ region of TEM-β-lactamase encoding its signalsequence was amplified from plasmid pBR322 by PCR with primers B1(5′-ATA GAA TTC AAA AAG GAA GAG TAT G-3′) containing an EcoRI site, andprimer B2 (5′-CTG GGG ATC CAA AAA CAG GAA GGC-3′) containing a BamHIsite. The amplified PCR fragment was cleaved with BamHI and EcoRI andinserted into pUC19′phoD, cleaved with the same restriction enzymes,resulting in plasmid pUC19bla-phoD. For easy selection of recombinantclones plasmid pOR124, containing a tetracycline resistance gene wasinserted 3′ of the bla-phoD gene fusion using an unique PstI site. Fromthe resulting plasmid pUC19bla-phoD-Tc an EcoRI-BglII fragmentcontaining bla-phoD and the tetracycline resistance gene of pOR124 wasisolated and inserted into pMUTIN2 cleaved with EcoRI and BamHI. Atplasmid pMutin2bla-phoD the bla-phoD gene fusion is under control of theIPTG-inducible P_(SPAC) promoter.

To construct a gene fusion consisting of the signal sequence of phoD andlacZ, a DNA fragment encoding the signal peptide of PhoD and thetranslational start site of phoD was amplified by PCR with primer P1containing a BamHI site and primer P4 (5′-GAG AAG GTC GAC GCA GCA TTTACT TCA AAG GCC CC-3′) containing a SalI site, and inserted into thecorresponding sites of pOR124 resulting in plasmid pOR124phoD′. Next thelacZ gene lacking nine 5′ terminal codons was amplified using primers L1(5′-ACC GGG TCG ACC GTC GTT TTA CAA CG-3′) containing a SalI site andprimer L2 (5′-GGG AAT TCA TGG CCT GCC CGG TT-3′) containing an EcoRIsite and subsequently inserted into the corresponding sites ofpOR124phoD. The resulting plasmid pOR124phoD-lacZ was linearized withBamHI and inserted into pAR3 cleaved with BglII. The resulting plasmidpAR3phoD-lacZ allows the arabinose inducible expression of the phoD-lacZgene fusion.

To obtain a plasmid mediating an inducible overexpression of tatA_(d)tatC_(d) of B. subtilis, the DNA region containing these genes includingtheir ribosome binding sites was amplified by PCR with the primers T1(5′-CAA GGA TCC CGA ATT AAG GAG TGG-3′) containing a BamHI site andprimer T2 (5′-GGT CTG CAG CTG CAC TAA GCG GCC GCC-3′) containing a PstIsite. The amplified fragment was cleaved with BamHI and PstI and clonedinto the corresponding sites of pQE9 (QIAGEN), resulting inpQE9tatA_(d)/C_(d).

To obtain TG1 ΔtatABCDE, plasmids pFAT44 and subsequently PFAT126covering in-frame deletions of E. coli tatE and tatABCD genes,respectively, were transferred to the chromosome of TG1 as described.Mutant strain TG1 ΔtatABCDE was verified phenotypically by mutant cellseptation phenotype, hypersensitivity to SDS and resistance to P1 phagesas described (Stanley et al. (2001) Escherichia coli strains blocked inTat-dependent protein export exhibit pleiotropic defects in the cellenvelope. J. Bacteriol. 183, 139-144).

SDS-PAGE and Western blot analysis—SDS-polyacrylamide gelelectrophoresis (SDS-PAGE) was carried out as described by Laemmli(Laemmli, U.K. (1970) Cleavage of structural proteins during assembly ofthe head of bacteriophage T4. Nature, 227, 680-685). After separation bySDS-PAGE, proteins were transferred to a nitrocellulose membrane(Schleicher and Schiill) as described by Towbin et al. (Towbin et al.(1979) Electrophoretic transfer of proteins from polyacrylamide gels tonitrocellulose sheets: procedure and some applications. Proc. Natl.Acad. Sci. USA, 76, 4350-4354). Proteins were visualized using specificantibodies against PhoD (16), LacZ (5PRIME-3PRIME, Boulder, USA) andSecB (laboratory collection) and alkaline phosphatase-conjugated goatanti-rabbit antibodies (SIGMA) according to the manufacturer'sinstructions.

Protein-chase experiments, immunoprecipitation and quantification ofprotein—Pulse-labelling experiments of E. coli strains were performed asdescribed earlier (Mililer et al. (1992) Suppression of the growth andexport defects of an Escherichia coli secA(Ts) mutant by a gene clonedfrom Bacillus subtilis. Mol. Gen. Genet. 235, 89-96). Cultures werepulse labelled with 100 μCi [³⁵S]-methionine, chased with unlabelledmethionine and samples were taken at the times indicated immediatelyfollowed by precipitation with trichloracetic acid (0° C.). After celllysis proteins were precipitated with specific antibodies against PhoD(Miller, J. P. and Wagner, M. (1999) Localisation of the cellwall-associated phosphodiesterase PhoD of Bacillus subtilis. FEMSMicrobiol. Lett., 180, 287-296) or β-lactamase and β-galactosidase(5PRIME-3PRIME, Boulder, USA). Relative amounts of radioactivity wereestimated by using a PhosphoImager (Fuji) and associated imageanalytical software PC-BAS.

In vivo protease mapping—In vivo protease mapping was carried outaccording to Kiefer et al. (EMBO J. (1997) 16, 2197-2204). Forspheroplast formation, cells were grown in TY-medium to exponentialgrowth. For induction of gene expression the medium was supplementedwith arabinose (0.2%) and/or IPTG (1 mM) for 60 min. After spheroplastformation cells were treated with proteinase K (SIGMA), with proteinaseK and Triton X-100 or remained untreated. Detection of cytosolic SecBrevealed the proteinase K resistance of Triton X-100 untreatedspheroplasts.

Determination of α-galactosidase activity—The assay and the calculationof β-galactosidase units (expressed as units per OD₆₀₀) were carried outas described by Miller ((1972) Experiments in molecular genetics. ColdSpring Harbor Laboratory, Cold Spring Harbor, N.Y.) using2-Nitrophenyl-β-D-galactopyranoside (ONPG, Serva). Enzymatic activity ofthe supernatant of lysozyme treated spheroplasts reflected theperiplasmic space. Activity associated to the spheroplasts representedthe cytosolic and cytoplasms membrane bound activity.

PhoD is not transported in E. coli —The initial aim was to test whetherPhoD could be exported by the Tat pathway in E. coli. For this purpose,we placed the encoding this peptide under the control of the P_(BAD)promoter of Salmonella typhimurium localized at plasmid pAR3. Theresulting plasmid allowed the arabinose-inducible enzymatically activeproduction of PhoD in E. coli TG1 (data not shown). Sincephosphodiesterase is highly toxic for the cell physiology of E. coliimmediately after induction of phoD expression cell growth ceased. Inorder to quantify transport of PhoD in E. coli TG1 (pARphoD) pulse-chaseexperiments were performed. As shown in FIG. 8, no processing of thewild-type prePhoD was observed even after 60 min chase, indicating thatprePhoD was not translocated by the E. coli Tat machinery. Localisationof PhoD was further localised by in vivo protease mapping. As shown inFIG. 8 prePhoD was not accessible to. Proteinase K at the outer side ofthe cytosolic membrane, demonstrating that PhoD remains in a cytosoliclocalisation in E. coli TG1 (pARphoD).

PhoD can be transported via the Sec-dependent protein translocationpathway—Absence of prePhoD processing in E. coli could be due toinefficient recognition of the signal peptide of PhoD by the E. coliTat-machinery or due to the nature of the mature part of the PhoDpeptide. This B. subtilis protein could have unexpected foldingcharacteristics or necessity of co-factors not present in E. coli. Inorder to address this question, the DNA encoding the mature peptide ofPhoD was fused to the region encoding the signal peptide ofTEM-β-lactamase (SP_(Bla)). The resulting gene fusion was cloned intothe pMUTIN2 vector containing an IPTG-inducible P_(SPAC) promoterallowing the synthesis of the SP_(Bla)-PhoD peptide. The transport andprocessing of this fusion protein was analysed by immunoblotting ofwhole cell extracts of E. coli strain TG1 (pMUTIN2bla-phoD). As shown inFIG. 8A, lane 2, SP_(Bla)-PhoD was completely converted to a proteinwith a molecular weight of mature PhoD indicating the efficienttransport of the protein. In order to elucidate the export path used forSP_(Bla)-PhoD translocation, Sec-dependent transport was selectivelyinhibited by addition of sodium azide (3 mM). While presence of sodiumazide abolished conversion of SP_(Bla)-PhoD to PhoD addition ofnigericin did not retard processing of SP_(Bla)-PhoD (FIG. 8A, lanes 3and 4). To analyse Sec-dependence of SP_(Bla)-PhoD transport moredetailed, expression of bla-phoD in E. coli TG1 (pMUTIN2bla-phoD) wasinduced in presence or absence of sodium azide, pulse-labelled with[³⁵S]-methionine and PhoD was subsequently immunoprecipitated. FIG. 8Bdemonstrates the kinetics of conversion of SP_(Bla)-PhoD to mature PhoD.Presence of sodium azide significantly retarded maturation ofSP_(Bla)-PhoD (FIG. 8C). These data indicate that PhoD can betransported in E. coli Sec-dependent. Thus, it can be concluded that thesignal peptide less PhoD peptide is not canalising the export route anddoes not prevent efficient transport or processing.

The signal peptide of PhoD can not mediate transport of LacZ in E. coliwild type cells—It has been shown that signal peptides containing a twinarginine motif can canalise transport of heterologous proteins via theTat-dependent translocation route (reviewed in Wu et al. (2000)Bacterial twin-arginine signal peptide-dependent protein translocationpathway: evolution and mechanism. J. Mol. Microbiol. Biotechnol 2,179-189). The signal peptide of the E. coli TMAO reductase (TorA) hasbeen successfully used to mediate Tat-dependent transport of thethylakoidal protein 23K, the glucose-fructose oxidoreductase GFOR ofZymomonas mobilis and the green fluorescent protein GFP. Other reportsindicated that Tat-signal peptides could determine the specificity ofthe Tat-dependent transport (Wu, supra). So could GFOR not betranslocated in E. coli (28).

To test whether the signal peptide of PhoD is recognised by the E. coliTat machinery and could canalise the transport of a protein in E. coli,we constructed a gene fusion consisting of the DNA region encoding the56 amino acid residues of PhoD signal peptide (SP_(PhoD)) and the lacZgene encoding β-galactosidase as a reporter protein. The gene hybrid wasinserted into plasmid pAR3 resulting in plasmid pAR3phoD-lacZ. Inductionof production of the SP_(PhoD)-LacZ fusion protein in E. coli TG1resulted in LacZ⁺ colonies (data not shown). Hence, correct folding andtetramerisation of the peptide as a prerequisite for its activity doesoccur in E. coli.

To analyse if the signal peptide of PhoD could mediate translocation ofLacZ into an extracytosolic localisation, enzymatic activity of LacZ wasmonitored in E. coli TG1 (pAR3phoD-lacZ). As shown in table II themajority of LacZ activity remained in the cytosol or the cytosolicmembrane. Since absence of enzymatic LacZ activity could be a result ofinefficient folding rather than absence of transport, we next studiedlocalisation of LacZ by using in vivo protease mapping. As shown in FIG.9A no processing of SP_(PhoD)-LacZ could be observed. The SP_(PhoD)-LacZfusion protein was not susceptible to protease digestion inspheroplasts. When spheroplasts were destroyed by addition of TritonX-100, the unprocessed SP_(PhoD)-LacZ protein became protease sensitive(FIG. 9A, lane 3). The reliability of the method was verified by usingthe cytosolic protein SecB as internal control (FIG. 9A). Inspheroplasts SecB was resistant to proteinase K, but was digested aftersolubilising the spheroplasts with Triton X-100.

Export of SP_(PhoD)-LacZ fusion protein in E. coli needs presence of theB. subtilis TatAd and TatCd transport components. The data demonstratedabove indicate that the Tat system of E. coli does not mediate transportof prePhoD or of the SP_(PhoD)-LacZ fusion protein. Absence oftranslocation could be due to the necessity of additional components forthe translocation of PhoD present only in B. subtilis or due to thespecificity of recognition of PhoD as a Tat-dependent substrate. Ourprevious observation that only the TatCd protein but not the second copyof TatC could mediate the Tat-dependent transport in B. subtilis was afirst indication for a specific recognition of prePhoD. To test thishypothesis, the B. subtilis tatA_(d)/C_(d) gene pair was amplified fromthe chromosome of B. subtilis and inserted under the control of theIPTG-inducible promoter of pQE9 (QIAGEN). The resulting plasmidpQE9tatA_(d)/C_(d) and the repressor plasmid pREP4 were transformed intoE. coli TG1 (pARphoD) and TG1 (pARphoD-lacZ).

In order to study the effect of TatA_(d)/C_(d) proteins on localisationof PhoD, strain TG1 (pARphoD, pREP4, pQE9tatA_(d)/C_(d)) expression ofphoD as well as tatA_(d)/C_(d) was induced with arabinose and IPTG.Unexpectedly, no PhoD could be detected in strain TG1 (pARphoD, pREP4,pQE9tatA_(d)/C_(d)) using Western blotting (data not shown). Inductionof TatA_(d)/C_(d) proteins in strain TG1 (pARphoD, pREP4,pQE9tatA_(d)/C_(d)) resulted in stable co-production of TatA_(d)/C_(d)proteins and the SP_(PhoD)-LacZ fusion protein (data not shown).SP_(PhoD)-LacZ processing was analysed in presence and absence ofTatA_(d)/C_(d) using pulse-chase labelling and subsequentimmunoprecipitation with specific antibodies against LacZ: While in TG1(pARphoD′-LacZ) no processing of SP_(PhoD)-LacZ could be observed (FIG.10A), in strain TG1 (pARphoD, pREP4, pQE9tatA_(d)/C_(d)) the peptide wasat least partially processed (FIG. 10B).

Since processing of the translocation product is an indication ofmembrane translocation but does not necessarily prove that export of theprotein has occurred, we examined whether LacZ could be localised in theperiplasmic space in TG1 (pARphoD, pREP4, pQE9tatA_(d)/C_(d)). As shownin table II the relative amount of periplasmic LacZ activity wassignificantly raised when compared to TG1 (pARphoD′-lacZ). Surprisingly,relative activity of LacZ in the strain expressing tatA_(d)/C_(d) wasmuch lower than compared to that of TG1 (pARphoD′-lacZ). To monitorlocalisation of the LacZ peptide, cells of strain TG1 (pARphoD, pREP4,pQE9tatA_(d)/C_(d)) were converted to spheroplasts, and treated withProteinase K. As shown in FIG. 10B co-expressing tatA_(d)/C_(d) thefusion protein SP_(PhoD)-LacZ was completely susceptible to proteasedigestion in spheroplasts. The resistance of SecB to the proteolyticdigestion confirms the reliability of the method. Unexpectedly, both theprocessed form and the precursor of the fusion protein were accessibleto the protease treatment. These results clearly show that theSP_(PhoD)-LacZ fusion protein is exported into the periplasmic space ofE. coli when the B. subtilis tatA_(d)/C_(d) genes are co-expressed.

TatA_(d)/C_(d)-mediated transport of SP_(PhoD)-LacZ needs delta pHdependent gradient at the cytosolic membrane and is Sec-independent—Todirectly proof that the membrane translocation of the system isdependent on the pH gradient across the cytosolic membrane, Sec- andTat-dependent protein translocation pathways were selectively blocked.Nigericin, an ionophore inhibiting the Tat-dependent proteintranslocation as a result of destroying the membrane potential (29), didefficiently block both, processing and translocation of SP_(PhoD)-LacZin TG1 (pARphoD′-lacZ, pREP4, pQE9tatA_(d)/Cd) (FIG. 11A). Sodium azide(3 mM), which severely inhibits Sec-dependent protein export byinterfering with the translocation-ATPase activity of the SecA protein(30), did not affect the localisation and the processing of theSP_(PhoD)-LacZ fusion protein in this strain as shown in FIG. 11B.

TatA_(d)/C_(d)-mediated transport of SP_(PhoD)-LacZ is not assisted byE. coli Tat components—Despite the above observations it can not beexcluded that the E. coli Tat machinery assists TatA_(d)/C_(d)-mediatedtransport of SP_(PhoD)-LacZ. The E. coli tat genes are constitutivelyexpressed in E. coli and therefore form a functional constitutivetranslocase unit (Jack et al. (2001) Constitutive expression ofEscherichia coli tat genes indicates an important role for thetwin-arginine translocase during aerobic and anaerobic growth. J.Bacteriol 183, 1801-1804). To exclude co-operative action of B. subtilisand E. coli Tat proteins, E. coli strain TG1 was deleted for tatABCDEgenes and subsequently transformed with plasmids pARphoD′-lacZ, pREP4and pQE9tatA_(d)/C_(d). Processing and localization of theSP_(PhoD)-LacZ fusion protein was analysed under identical conditions asdescribed for the E. coli tat+ strain. Despite the fact that the totalamount of LacZ found in the periplasmic fraction was reduced thancompared to the E. coli tat wild type strain expressing phoD′-LacZ andtatA_(d)/C_(d), the relative amount of periplasmic LacZ wassignificantly elevated than compared to TG1 (pARphoD′-LacZ) (Table II).As shown in FIG. 12 in absence of the E. coli tatABCDE genes most of theSP_(PhoD)-LacZ hybrid protein was protease accessible demonstrating theextracytosolic localisation of SP_(PhoD)-LacZ. The resistance of SecB tothe proteolytic digestion demonstrated the stability of the spheroplasts(FIG. 13). Surprisingly, no processing of the SP_(PhoD)-LacZ fusionprotein could be observed in absence of tatABCDE. Taken together, the B.subtilis Tat components TatAd/Cd can mediate translocation of the hybridpeptide consisting of the twin-arginine signal peptide of PhoD and LacZ.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity andunderstanding, it will be obvious that certain changes and modificationsmay be practiced within the scope of the appended claims.

TABLE I Predicted Twin-Arginine Signal Peptides of B. subtilis*

*Putative twin-arginine signal peptides were identified in two ways.First, the presence of the consensus sequence R-R-X-φ-φ (φ is ahydrophobic residue), immediately in front of an amino-terminalhydrophobic region as predicted with the TopPred2 algorithm (34, 35),was determined. To this purpose, the first 60 residues of all annotatedproteins of B. subtilis in the SubtiList database(http://bioweb.pasteur.fr/Genolist/Subtilist.html) were used. Second,within the group of twin-arginine membrane sorting signals, cleavablesignal peptides were identified with the SignalP algorithm (61, 62).Conserved residues of the twin-arginine consensus sequence (R-R-X-φ-φ)are indicated in bold. In addition, positively charged residues thatcould function as a so-called Sec-avoidance signal (54) are indicated inbold and italics. The hydrophobic H-domain is indicated in gray shading.In signal peptides with a predicted signal peptidase I cleavage site,residues from position −3 to −1 relative to the signal peptidase Icleavage site are underlined. Notably, some of these proteins containone or more putative transmembrane segments elsewhere in the protein(indicated with “TM”), or are putative lipoproteins. Residues forming aso-called lipobox for signal peptidase II cleavage are enlarged in size.

TABLE II Plasmids and Strains Relevant properties Reference PlasmidspUC21 cloning vector; 3.2 kb; Ap^(r) 63 pJCd1 pUC21 derivative; carryingthe tatCd gene; 5.4 kb; Ap^(r) This work pJCd2 pUC21 derivative for thedisruption of tatCd; 6.3 kb; Ap^(r); Km^(r) This work pJCy1 pUC21derivative; carrying the tatCy gene; 5.3 kb; Ap^(r) This work pJCy2pUC21 derivative for the disruption of tatCy, 6.5 kb; Ap^(r); Sp^(r)This work pMutin2 pBR322-based integration vector for B. subtilis;containing a 31 multiple cloning site downstream of the Pspac promoter,and a promoter-less lacZ-gene preceded by the RBS of the spoVG gene; 8.6kb; Ap^(r); Em^(r) pMlCd1 pMutin2 derivative; carrying the 5′ part ofthe B. subtilis tatCd This work gene pMlCy1 pMutin2 derivative; carryingthe 5′ part of the B. subtilis tatCy This work gene pDG792 contains a Kmresistance cassette; 4.0 kb; Ap^(r), Km^(r) 64 pDG1726 contains a Spresistance cassette; 3.9 kb; Ap^(r), Sp^(r) 64 Strains E. coli MC1061F⁻; araD139; Δ (ara-leu)7696; Δ (lac)X74; galU; galK; hsdR2; 65 mcrA;mcrB1; rspL B. subtilis 168 trpC2 2 ΔtatCd trpC2; tatCd; Km^(r) Thiswork ΔtatCy trpC2; tatCy, Sp^(r) This work ItatCd trpC2; Pspac-tatCd;tatCd-lacZ; Em^(r) This work ItatCy trpC2; Pspac-tatCy, tatCy-lacZ;Em^(r) This work ΔtatCd-ΔtatCy trpC2; tatCd, Km^(r); tatCy, Sp^(r) Thiswork ItatCd-ΔtatCy trpC2; Pspac-tatCd; tatCd-lacZ; Em^(r); tatCy; Sp^(r)This work

TABLE III β-galactosidase activity (U/OD₆₀₀)*. strain LPDM MM SSM TY 1680 0.1 ± 0.1 0.3 ± 0.2 0.6 ± 0.2 ItatCd 1.1 ± 0.7 0.1 ± 0.1 0.3 ± 0.2 0.5± 0.2 ItatCy 6.1 ± 2.5 10.0 ± 3.6  4.0 ± 2.0 13.2 ± 5.5  *To investigatethe transcription of the tatCd and tatCy genes, cells of B. subtilisItatCd (tatCd-lacZ), ItatCy (tatCy-lacZ) or the parental strain 168 (nolacZ gene fusion) were grown for 10 hours in LPDM, MM, SSM or TY mediumafter dilution from an overnight culture. Samples for β-galactosidaseactivity determinations were taken at hourly intervals, starting 4 hoursafter dilution from the overnight culture. As the β-galactosidaseactivities showed little variation during the entire period of sampling,average values were determined. The numbers in the table representaverage values from 3 different experiments. Note that HPDM medium wasused for the overnight culture of cells grown in LPDM medium, whileovernight cultures of cells grown in MM, SSM or TY medium were preparedwith the respective media.

TABLE IV Twin-Arginine Signal Peptides of PhoD and PhoD-like proteins*protein signal peptide PhoD (Bsu)

SP1 (Sco)

SP2 (Sco)

SP3 (Sco)

SP4 (Ste)

*Homologues of B. subtilis PhoD were identified by amino acid sequencesimilarity searches in GenBank using the BLAST algorithm. SP1 (Sco),gene SCC75A.32c of Streptomyces coelicolor (CAB61732); SP2 (Sco), geneSCF43A.18 of S. coelicolor (CAB48905); SP3 (Sco), gene SC4G6.37 of S.coelicolor (CAB51460), and SP4, phoD gene of Streptomyces tendae(CAB62565). GenBank accession numbers are indicated in parentheses.Conserved residues of the twin-arginine consensus sequence are

TABLE 5 Plasmids and Strains Relevant properties Reference Plasmids pAR3pACYC184 derived plasmid carrying the araB promoter 25 operator and thearaC repressor gene from Salmonella typhimurium; Cm^(r a) pAR3phoD pAR3derivative; carrying the phoD gene; Cm^(r) This work pAR3phoD-lacZ pAR3derivative; carrying a fusion gene consisting of This work the signalsequence region of phoD and lacZ; Cm^(r) pQE9 pBR322-based vector forIPTG-inducible synthesis of Qiagen His₆-tagged proteins; Ap^(r) pREP4plasmid; containing lacl^(q) represser gene; Km^(r) Qiagen pORI24plasmid; replicates only in E. coli rep⁺ strains; Tc^(r) 37 pMUTIN2pBR322-based integration vector for B. subtilis; 38 containing amultiple cloning site downstream of the Pspac promoter, and apromoter-less lacZ-gene preceded by the RBS of the spoVG gene; Ap^(r);Em^(r) pMUTIN2bla-phoD PMUTIN2 derivative; carrying a fusion geneconsisting This work of signal sequence region of bla and phoDpQE9tatA_(d)/C_(d) pQE9 derivative; carrying the B. subtilistatA_(d)/C_(d) genes This work pFAT44 pMAK705 (Hamilton et al., 1989)derivative plasmid 7 containing in frame deletion of E. coli tatEpFAT126 pMAK705 derivative plasmid containing in frame 39 deletion of E.coli tatABCD Strains E. coli TG1 F⁻ araD139 Δ(ara-leu)7696 Δ (lac)X74galU galK 40 hsdR2 mcrA mcrB1 rspL TG1 ΔtatABCE TG1 ΔtatABCE This workB. subtilis 168 trpC2 13 ^(a) Cm^(r), chloramphenicol resistance marker;Ap^(r), ampicillin resistance marker, Km^(r), kanamycin resistancemarker; Tc^(r), tetracycline resistance marker; Em^(r), erythromycinresistance marker

TABLE 6 Localisation of β-galactosidase activity in E. coliTG1(pAR3phoD-lacZ) strains. LacZ activity (units/ OD₆₀₀) strain cellbound periplasmic total activity % export TG1(pAR3phoD-lacZ) 1108 +/−201 67 +/− 5 1175  6.4 +/− 3.4 TG1(pAR3pAoD-lacZ, 226 +/− 11 94 +/− 2320 29.4 +/− 0.4 pREP4, pQE9tatA_(d)/C_(d)) TG1 ΔtatABCE (pAR3phoD-lacZ,278 +/− 8  39 +/− 5 317 12.5 +/− 0.9 pREP4, pQE9tatA_(d)/C_(d)) Toinvestigate the translocation of the hybrid protein consisting of SPphoDand LacZ, cells of E . coli strains were grown in TY medium toexponential growth. Samples for β-galactosidase activity determinationswere taken from supernatants of lysozyme treated cells representingperiplasmic activity and spheroplasts representing cell bound activity.Experiments were carried out with duplicated cultures. +/-, standarddeviation.

1. An isolated chimeric polypeptide comprising (i) a Bacillus subtilisPhoD secretion signal peptide and (ii) a heterologous polypeptide. 2.The chimeric polypeptide of claim 1, wherein said heterologouspolypeptide is not naturally associated with a secretion signal peptide.3. The chimeric polypeptide of claim 1, wherein the heterologouspolypeptide is an enzyme.
 4. The chimeric polypeptide of claim 3,wherein the enzyme is selected from the group consisting of a protease,cellulase, amylase, lipase, and hydrolase.
 5. The chimeric polypeptideof claim 4, wherein the enzyme is a protease.
 6. The chimericpolypeptide of claim 4, wherein the enzyme is a cellulase.
 7. Thechimeric polypeptide of claim 4, wherein the enzyme is an amylase. 8.The chimeric polypeptide of claim 4, wherein the enzyme is a lipase. 9.The chimeric polypeptide of claim 4, wherein the enzyme is a hydrolase.10. The chimeric polypeptide of claim 2, wherein the heterologouspolypeptide is an enzyme.
 11. The chimeric polypeptide of claim 10,wherein the enzyme is selected from the group consisting of a protease,cellulase, amylase, lipase, and hydrolase.
 12. The chimeric polypeptideof claim 11, wherein the enzyme is a protease.
 13. The chimericpolypeptide of claim 11, wherein the enzyme is a cellulase.
 14. Thechimeric polypeptide of claim 11, wherein the enzyme is an amylase. 15.The chimeric polypeptide of claim 11, wherein the enzyme is a lipase.16. The chimeric polypeptide of claim 11, wherein the enzyme is ahydrolase.